A lego sculpture is copyrighted. Lego blocks are not. The threshold between blocks and sculpture is not well-defined, but if an AI isn't prompted specifically to attempt to mimic an existing work, its output will be safely on the non-copyrighted side of things.
A derivative work is separately copyrightable, but redistribution needs permission from the original author too. Since that usually won't be granted or would be uneconomical, the derivative work can't usually be redistributed.
AI-produced material is inherently not copyrightable, but not because it's a derivative work.
I dispute the idea that token sequences reproduced from the model are not derived works.
I predict, no pun intended, that a time is coming when the idea that it's not a derived work will be challenged in mainstream law.
The slop merchants are getting a free ride for the time being.