I don’t think that’s true. It certainly did give them more weight, but I don’t see that that was the purpose of the EC. The EC was proposed in The Federalist Papers #63, written by Alexander Hamilton (edit: This is wrong. This article was written in an attempt to convince people that the EC in the proposed constitution was a good idea. The EC proposal came about as a result of a bunch of proposals, counter-proposals, and attempts to balance the wishes of big states, small states, popular vote vs. appointment by legislature, and, yes, free states and slave states). Hamilton was an abolitionist, so it seems unlikely that he would have proposed a solution to benefit slave states. He says:

It was desirable that the sense of the people should operate in the choice of the person to whom so important a trust was to be confided. This end will be answered by committing the right of making it, not to any preestablished body, but to men chosen by the people for the special purpose, and at the particular conjuncture.

It was equally desirable, that the immediate election should be made by men most capable of analyzing the qualities adapted to the station, and acting under circumstances favorable to deliberation, and to a judicious combination of all the reasons and inducements which were proper to govern their choice. A small number of persons, selected by their fellow-citizens from the general mass, will be most likely to possess the information and discernment requisite to such complicated investigations.

He actually favored a direct popular vote to elect the President and said as much on July 19, 1787.

In Madison’s view, this wouldn’t work at that time because the right to vote was more widespread in the North (for example, MA had universal suffrage, even for free blacks, and no property owning requirement like in the south), and instituting a popular vote would also deprive the South of the influence it would have because of its slave population (slaves obviously were not entitled to vote, and ultimately, as a compromise, were made to count as 3/5ths of full person, with the Electoral College allowing slaveholders to vote on their behalf):

Mr. MADISON. If it be a fundamental principle of free Govt. that the Legislative, Executive & Judiciary powers should be separately exercised, it is equally so that they be independently exercised. There is the same & perhaps greater reason why the Executive shd. be independent of the Legislature, than why the Judiciary should: A coalition of the two former powers would be more immediately & certainly dangerous to public liberty. It is essential then that the appointment of the Executive should either be drawn from some source, or held by some tenure, that will give him a free agency with regard to the Legislature. This could not be if he was to be appointable from time to time by the Legislature. It was not clear that an appointment in the 1st. instance even with an eligibility afterwards would not establish an improper connection between the two departments. Certain it was that the appointment would be attended with intrigues and contentions that ought not to be unnecessarily admitted. He was disposed for these reasons to refer the appointment to some other source. The people at large was in his opinion the fittest in itself. It would be as likely as any that could be devised to produce an Executive Magistrate of distinguished Character. The people generally could only know & vote for some Citizen whose merits had rendered him an object of general attention & esteem. There was one difficulty however of a serious nature attending an immediate choice by the people. The right of suffrage was much more diffusive in the Northern than the Southern States; and the latter could have no influence in the election on the score of the Negroes. The substitution of electors obviated this difficulty and seemed on the whole to be liable to fewest objections.

After the initial Electoral College proposal (his second favorite option) was rejected, Madison again argued for a popular vote on July 25, 1787:

Mr. MADISON: There are objections agst. every mode that has been, or perhaps can be proposed [for electing the President]. The election must be made either by some existing authority under the Natil. or State Constitutions — or by some special authority derived from the people — or by the people themselves.

The option before us then lay between an appointment by Electors chosen by the people — and an immediate appointment by the people. He thought the former mode free from many of the objections which had been urged agst. it, and greatly preferable to an appointment by the Natl. Legislature. As the electors would be chosen for the occasion, would meet at once, & proceed immediately to an appointment, there would be very little opportunity for cabal, or corruption. As a farther precaution, it might be required that they should meet at some place, distinct from the seat of Govt. and even that no person within a certain distance of the place at the time shd. be eligible. This Mode however had been rejected so recently & by so great a majority that it probably would not be proposed anew. The remaining mode was an election by the people or rather by the qualified part of them, at large: With all its imperfections he liked this best. He would not repeat either the general argumts. for or the objections agst. this mode. He would only take notice of two difficulties which he admitted to have weight. The first arose from the disposition in the people to prefer a Citizen of their own State, and the disadvantage this wd. throw on the smaller States. Great as this objection might be he did not think it equal to such as lay agst. every other mode which had been proposed. He thought too that some expedient might be hit upon that would obviate it. The second difficulty arose from the disproportion of qualified voters in the N. & S. States, and the disadvantages which this mode would throw on the latter. [Note that this is not a disproportion in population, but in “qualified voters”.] The answer to this objection was 1. that this disproportion would be continually decreasing under the influence of the Republican laws introduced in the S. States, and the more rapid increase of their population. [i.e., the right to vote would be extended in the South under “the influence of … republican laws”] 2. That local considerations must give way to the general interest. As an individual from the S. States he was willing to make the sacrifice.

The Electoral College allowed (populous) slaveholding southern states to not expand the franchise under the influence of republican laws without paying a political price for it.

I didn’t say it was illegitimate. I said the argument was offered ex post–after the fact–to justify the EC. I also think it’s wrong, and provably so–one of those theoretical arguments that doesn’t stand up to an empirical test.

The founding fathers on the dangers of foreign interference and unscrupulous men

The Mode of Electing the President From the New York Packet. Friday, March 14, 1788.

These most deadly adversaries of republican government might naturally have been expected to make their approaches from more than one querter, but chiefly from the desire in…