That’s not what the case is about.
The states are arguing that the electors are bound to vote based on the state’s laws....whatever they may be. In other words, the electors have to vote how the state wants them to vote and not vote based on their own free will.
So the state could pass a law that says its electors are bound to the winner of the national vote. It would then be illegal for an elector to instead vote for the candidate that won their state.
The Constitution is very specific on how an elector is chosen:
Article IISection 1. The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice President, chosen for the same Term, be elected, as follows Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.
And the Twelfth Amendment lays out how the Electors cast their vote:
The Electors shall meet in their respective states, and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves; they shall name in their ballots the person voted for as President, and in distinct ballots the person voted for as Vice-President, and they shall make distinct lists of all persons voted for as President, and of all persons voted for as Vice-President, and of the number of votes for each, which lists they shall sign and certify, and transmit sealed to the seat of the government of the United States, directed to the President of the Senate;
But the Constitution is silent on who an elector must vote fo.
So the Interstate Compact could still go fowrard.
The message here is to nominate your electors wisely.