If you think about it, plagiarism is just using a source so well that derivated work becomes indistinguishable from the actual source. Your third grade self wouldn't really be able to do that.

Using a more realistic example, if you went to Nerd & Jock (or any artist) page on Instagram, learned their style perfectly (as AI can do), and started to create/sell comics with this style, people would probably still call it plagiarism, wouldn't they?

I imagine that this possibility is what most artists fear and why they say that every model is using "stolen art", even if they don't articulate it very well.