> that the reading that the reading of the commerce clause to mean that regulating interstate commerce is exclusively the province of the federal government (which it doesn't explicitly say) is.

Again, that's not really a partisan position either. If anything, there's an argument to be made (as has been made elsewhere in the comments) that this would be more of an ideologically liberal position to take.

AEI is being very strategic here: they've found a legal position that is wholeheartedly aligned with mainstream liberal and left-wing ideology and which is supported by a century of case law, and they're using that position to advocate their favored outcome. The outcome notwithstanding, the legal position itself is emphatically not a partisan one, nor is it one that is particularly conservative ideologically.

Of course, that doesn't mean that AEI is correct, either legally or morally. They could still lose on other grounds. (Or, they could still win, despite you and me agreeing that their position is morally wrong).