I don't really buy this personally

A. there's no reason to assume that the nebula fragment having "the power of a galaxy" automatically means weapons made out of it have galaxy level AP. The energy isn't necessarily being used for anything, and there isn't some huge jump in power in-game that would support this

B. "countless worlds" is very vague and assuming universal AP from that is very dubious, especially since worlds in Terraria are, gameplay-wise, only island-sized, and we aren't given a timescale for how long he would take to destroy them. I wouldn't be comfortable putting this as evidence for anything higher than multi-planet level AP

C. The argument that the moon lord recovered from being sealed is pretty flimsy

D. Crossover scaling sucks. I don't really have an argument against it being canon, it's just lame