> what if code contributions objectively improve something?

If the contribution is complex enough, it is no longer an 'objective improvement' but rather a judgement call, and in the process becomes copyrightable. This is where the trouble lies, and why this kind of AI involvement is banned.

If it is not, for example by being a one-line fix that literally cannot be performed differently, it's a different story. Then it can be merged, viewed either as a menial change (exempt by the ban) or by transfer of ownership (the reviewer becomes the effective author) because it is not copyrightable.