In the end there is always a separate reviewer. Even if the producer is adversial and not even looking at the code then this process exists to catch it. This is not at all different from people attempting to get backdoors into Linux.

People using LLMs put up much larger changes, and much faster. Depending on the LLM used the quality is often much lower.

A lot of projects just stopped accepting any outside contributions rather than deal with the tidal wave of slop they were getting. Choosing to not accept LLM code is IMHO a better compromise.