Human artists use copy/paste as a tool all the time, and it doesn't kill their careers. The element you're missing here is that jychang wasn't talking about copying pieces verbatim from others' work without permission & attribution, but rather using it as a tool to manipulate your own art ("to copy/paste a layer"). Imagine being able to do the same but with an AI model working behind the scenes to adjust the object to fit the scene it's being pasted into.