The thing they want is clear legal standing. If your submission is majority LLM generated, you can't really confirm that "your" code isn't a copy of something in the training set that has an incompatible license (or a clear copyright violation). That might not be a big deal for some projects, but being this is GNU and their entire identity is centered around free software and licensing, it's kind of a big deal to them.

That's just not an issue in practice. All major AI labs match generated code against their training database to prevent outputting verbatim copies. They're so confident that they won't output copyrighted code that they offer copyright indemnification to paid users.