I don't think it's unconstitutional. The way you're saying, SCOTUS will look at the Constitution thinking of the intent, not the actual binding legal phrasing. But with the Compact, they'll look at the binding legalities, rather than the intent. I doubt they'd split it like that; either they'd go exact Constitutional phrasing, in which case they don't have the right to butt in and it's Constitutional, or they'll consider the intent, which is to make America more democratic, and decide it's a worthy enough cause. Or, of course, a mix (some judges might go for one, some for another).
It's not that. If it were that, it would be a either a straightforward division between the strict constructionists and the activists or a just as straightforward party line vote. It's rather the Court needing to protect its own perogatives. It has no way of permitting the Compact without also allowing states to give their electors to the champion pig lifter. Given a choice like this, despite how unlikely the pig-lifter compact is, the Court is likely choose to protect its own rights.