Stop trying to equate LLM-generated code with indexing-based autocomplete. They’re not the same thing at all: LLM-generated code is equivalent to code copied off Stack Overflow, which is also something you’d better not be attempting to fraudulently pass off as your own work.
For example, you either make your contributors attest that their changes are original or that they have the right to contribute their changes—or you assume this of them and consider it implicit in their submission.
What you (probably) don’t do is welcome contributions that the contributors do not have the right to make.
Assuring you didn’t include any AGPLv3 code in your contribution is exactly the same kind of assurance. It also doesn’t provide any provenance.
Conflating assurance with provenance is bogus because the former is about making a representation that, if false, exposes the person making it to liability. For most situations that’s sufficient that provenance isn’t needed.