You are correct, and my earlier comments were wrong. I simply assumed that they had structured it informally, but I now see that it is intended to be a true compact among states.
I read over their material in which they address Article 1 Section 10 of the Constitution but fail to make any kind of persuasive case that this compact would not require Congressional approval. The best they can do is cite a few Supreme Court opinions allowing compacts which either have implied consent or are effectively so straightforward and have so little effect on states outside the compact as to not need Congressional consent.
However, most compacts among states do receive explicit Congressional consent. I doubt very much if such a far-reaching one as this could get past the Supreme Court without such explicit Congressional consent. It obviously effects nonmembers states politically in a multitude of ways and on a core issue of our democracy.
So the question is, how likely would Congress be to grant such consent? A lot would depend upon which political party controlled each house of Congress. It could get bottled up in committees, and it could be filibustered in the Senate. Small states would tend to oppose it, since it eliminates their disproportionate influence in the Electoral College. And since small states also have disproportionate influence in the U.S. Senate, the tendency will be for them to block consent.
This is a serious movement with an array of legal advisors and constitutional advisors on their side. More than likely it would be challenged in the courts all the way up to the SCOTUS. There is no guarantee that this would be declared unconstitutional and SCOTUS may wish to stay out of it and defer to the states' authorty and rights under the Constitution. You can bet that the legal issues have been vetted in the various states, including California, where this plan is moving forward.