There are other reasons why a project like Zig might not want to accept LLM generated contributions.

Zig, as programming language, has a multiplier codebase. A bug may affect a significant larger portion of users than most libraries or binaries will, as it's a fundamental building block of everything that uses Zig. Just that could be worth the extra scrutiny on every individual commit.

There's also the usual arguments: copyright ethics, environmental ethics and maintainer burden.