The 10th Amendment - it is nowhere mentioned in the Constitution that the federal government is required to maintain any separation of religion and state - in fact, references to God are peppered throughout the federal government. Therefore, under the 10th, this was a state matter, to be resolved by the State of Alabama.
I'm not a Christian, but I think Moore is right here. There was no violation to any federal-level enumerated rights by having that monument in the state courthouse. So the feds create a right from thin air and use that to deny the Alabama Judiciary to place a monument in their own building. That's usurpation. And it's just one example of thousands over the last few decades.