Yes. If this decision holds up, it would be unconstitutional for Colorado to instruct it's electors to vote for the winner of the national popular vote. Hypothetical: Trump takes Colorado in 2020, but Biden wins the national popular vote. The Trump electors would not be bound by the Colorado law instructing them to cast their votes for Biden.
That said, I think the Compact is unconstitutional on other grounds as well, such as the Equal Protection one man one vote rule.