I'm not even an American, and I've heard all about the Getty Images Address.

I think you're technically right, but that this will be overlooked from a legal perspective because it's less obvious that humans have been training ourselves on the prior art of others. We tend to blend in additional things besides prior art. (eg. nature, sensations, etc.)

That's technically not a stock image, it's a portait that has been public domain for a long time.

But you've seen many PD images reshared by stock imagery companies. It raises the question of why false assertions of ownership aren't easily prosecuted, given that they constitute a kind of fraud upon the public.

That's what trademark law is. But I don't think the person who painted the portrait is going to be suing Getty for impugning his good reputation.

If something's public domain, anybody can use for anything they want, even if that's just rehosting it with your watermark.