“The Constitution leaves it up to the States to decide how to choose their Electors.”
True, but this is telling the Electors how they must vote.
It's no different than today.
The state legislature appoints electors who say they'll vote a certain way. In most cases for the winner of that state's popular vote.
Electors aren't required to vote any particular way, they just won't get appointed unless they agree to do the legislature's bidding.