There are more than a few constitutional legal scholars who disagree. Here are some articles expressing their opinions.
https://dailycaller.com/2019/02/04/natelson-popular-vote/
https://www.jurist.org/commentary/2012/02/william-ross-vote-compact/
They also don't say that they are going to go into a compact with other states to decide how they are going to pool or determine how their electors vote.
What there attempting to pass with their laws is how they are going to direct their electors to vote within their own State. There's nothing unconstitutional about that.
all the states currently have laws directing their electors on how they should vote. None of that is unconstitutional.
this will probably be the most difficult fight going forward, waking the sheeple up who live in the smaller States to see that this is not in their best interest to go along with this idea.