Each party submits a list of electors. I think the compact states will award the right to choose the states electors to the party based on the national vote rather than the states vote.
Colorado took it a step too far though, when their SoS tried to invalidate an elector's vote. "Faithless" electoral voters have existed for a very long time. Regardless of who picks the elector, according to this ruling the state cannot nullify that person's EC ballot. From the OP's link:
A U.S. appeals court in Denver said Electoral College members can vote for the presidential candidate of their choice and aren’t bound by the popular vote in their states.
The 10th U.S. Circuit Court of Appeals ruled Tuesday that the Colorado secretary of state violated the Constitution in 2016 when he removed an elector and nullified his vote when the elector refused to cast his ballot for Democrat Hillary Clinton, who won the popular vote.
So, it's the same problem whether a state signed up for the Pact or not, whoever is appointed to be an elector can vote or whoever they want to. And have done so in the past, although not too often. The real question, which this ruling did not address, is who gets to appoint those electors? Can a state award the elector appointments to the party of a person who did not win the state's popular vote if that person was the national popular vote winner?
The Constitution doesn't really say much about that, right? BUT - the apportioning of 2 US Senators to each state was done to avoid the possibility of the more populous states from running roughshod over the others. They knew then that votes in a heavily populated state would seem to count less than a vote in a sparsely populated one. Such has always been the case and up until lately it didn't matter cuz the person who won the national popular vote was almost always the electoral college winner too. So, will the SCOTUS essentially dismantle the EC in favor of the national plurality? It's interesting to speculate on what that would mean for this country's future if that happened.