I do not think some untested theory about sabotaging open-source projects by intentionally inserting copyrighted material using AI, causing legal issues for maintainers, is worthwhile to discuss here.

That is obviously not what anyone was referring to, nor does it make sense, when there is a much more reasonable basis to prohibit the same contribution.

Namely inserting vulnerabilities. This one actually happened before afaik, and provides a clear benefit to the attacker.