That's highly debatable. It is an interstate compact, yet it claims it will go into effect with Congressional consent. It also prohibits any member State from immediately withdrawing from the Compact. Today's decision says each State and DC decides if its electors are to be bound by a pledge. So if a member State wants to withdraw from the Compact, it can as soon as it wants.
Finally, by the Compact's terms, it doesn't go into effect unless States forming a majority of electoral votes sign on. Right now, not enough States have signed on and some Red States would have to sign on; that's unlikely to happen.
As of right now, the Compact is not in effect. If it ever gets enough States to join, it will be in court well beyond the election.
Its the interstate compact that is illegal.
.