Nothing is stopping you from using LLMs when contributing to their project (I think). One reason might simply be that they would rather spend the (very sparse) donation money on anything else but tokens.

https://asahilinux.org/docs/project/policies/slop/

They do currently ban LLM-assisted submissions. To be honest, even if LLMs are technically capable of writing code that assists the project, this at least helps keeps the 'floodgate' closed for certain low-quality PRs that other open-source projects are getting.