Is a painting not art because individual pigments aren't art?

I'm not sure if you're being sarcastic or not... I hope so.

Assuming you aren't, this is called a "false analogy" or "category error". Pigments are raw materials for painting in the same way that letters are raw materials for writing. Pigments make no claim to be art, never have. AI-generated text, on the other hand, is a fully-realized product that lacks the human intent that makes it art, but can be passed off as "art".