Most contributors generally respect the rules of a project, and there are other rules put in place that are also not technically enforceable, such as asking a contributor to not submit copyrighted code (this would be virtually impossible for a reviewer to distinguish if it came from a closed source project), which also gives the project a stronger defense if brought to court, as they can then at least point to the rules and show where a contributor willfully broke them. In comparison, I suspect a court would not look kindly on a policy that says "Don't disclose that you used a tool that has a 3 to 10% chance of introducing plagiarized copyrighted code with every prompt to avoid being shamed."

As an example, the WINE project has a rule to not allow anyone who has even seen Windows source code from contributing to the project. This is to avoid the chances of someone subconsciously or consciously submitting a reproduction of what they saw in the Windows source code, and thus hopefully prevent the project being sued into oblivion from Microsoft (who would likely love to see that happen).

Adding a stipulation in the guidelines that contributors can only use more-ethical LLMs likely wouldn't entirely prevent someone breaking that rule, but by in large, it is likely that most would adhere to it, making it an effective rule despite being difficult to enforce.