That is the opinion of one man on one issue. There were several proposals for choosing the president and there was very little support for popular election and they had nothing to do with slavery.
The following are some quotes from delegates about popular election.
Morris: In wondering whether the revolution has gotten out of hand warned “…the mob (has) begun to think and reason.”
Gerry: In discussing the country’s problems said “...the evil we experience flow from the excesses of democracy.”
Mason: “It would be as unnatural ... to let the people choose the president as it would be to refer a trial of colors to a blind man.”
Hamilton: “The voice of the people has been said to be the voice of God; and however generally this maxim has been quoted and believed, it is not true in fact. The people are turbulent and changing; they seldom judge right.”
Sherman: “The people immediately should have as little to do as may be about the government.”
Gouverneur Morris: Morris argued for property qualifications for the voters and stated “give the votes to the people who have no property and they will sell them to the rich who will be able to buy them.”
Dickinson: Dickinson considered property qualifications a “necessary defense against the dangerous influence of those multitudes without property and without principles, with which our country like all others, will in time abound.”
Some actually worried a popular vote would give too much power to the slave states because they could give slaves the vote which their owners could easily control.
Recent revisionist interpretations have given excessive influence to slavery throughout American history which have been debunked by the most respected historians for each period.