This is false for several different reasons.
There were two initial proposals for Congress: the New Jersey Plan and the Virginia Plan. (Virginia, of course, being the leading slave-owning state.) Under both plans, the President would have been elected by Congress. There were concerns that having Congress elect the President would jeopardize separation of powers. So the Electoral College was created, with one elector for each member of Congress. It had nothing to do with protecting the interests of slave-holding states, but instead was designed to make the Presidency more independent of Congress.
Indirect election of the executive is somewhat anti-majoritarian, but also extremely common. Justin Trudeau was not elected Prime Minister of Canada by the general population. He was elected by about 80,000 eligible members of the Liberal Party, and became Prime Minister when the people elected other members of his party to the Parliament. I will add that the leading proponent of direct election of the President and the Senate was Andrew Jackson, strongly opposed abolition and was generally a horrible person.
Now, the New Jersey Plan would have allocated one member of Congress to each state. It was also supported by New York, Delaware, and Connecticut. Of those, only Delaware had a significant enslaved population. The Virginia Plan created a two-chamber legislature with proportional representation based on population. Yes, slave-holding Virginia was the one that wanted proportional representation. The Connecticut Compromise that led to the current allocation really was about small versus large states, not slavery: https://en.wikipedia.org/wiki/1790_United_States_Census. Large states like Massachusetts and Pennsylvania had few to no enslaved persons, and small states like Georgia and Delaware had significant numbers of enslaved persons.
The 3/5 compromise was designed to reduce the interest of slave-holding states. The number originated in an amendment proposed to the Articles of Confederation, which proposed to set the tax obligation for each state to the federal government "in proportion to the number of inhabitants of every age, sex, and quality, except Indians not paying taxes." https://en.wikipedia.org/wiki/Three-fifths_Compromise#Confed.... Virginia complained that this would count enslaved persons, thus increasing Virginia's tax obligations. Eventually, they settled on counting 3/5 the number of enslaved persons towards the state's tax obligation. That same compromise came up later in the apportionment context.