Idk, I guess the point is that if text is 100% gpt written and not reviewed by a human, then there is a risk that gpt learns from bad gpt examples. If you review and modify it to remove the watermark, then it is effectively human reviewed/labelled content and ok for re-ingestion in future iterations.

But tbh the guys at openai are pretty capable, I'm sure they'll think of something. I don't know anything more than the headline I read.