Pretty much everything is trained on copyrighted content: machine translation software, TWDNE, DALL-E, and all the GPTs. Software people are bringing this up now because it's their ox being gored. It's the same as when furries got upset about This Fursona Does Not Exist.[1][2]
To expand on your argument, pretty much every person is trained on copyrighted content too. Doesn't make their generated content automatically subject to copyright either.