That is true, but if a state law specifies the electors are selected by a vote of the people, it will be challenged in court, and be forced to the SCOTUS.
The Constitutions says the state legislature alone gets to determine how electors are selected. Even the governor is not required to sign such decree.
It was the same process as electing Senators before the 17th Amendment.
But we know the SCOTUS can decide against the Constitution.
A state law does not override the U.S. Constitution, and there is nothing that legally prevents a state legislature from changing its process for selecting electors after Election Day.
In theory, this would probably only happen if the ballot collection and counting process in a state was so fouled up that a true winner could not be legitimately determined.