It's often quite easy to distinguish LLM-generated low-effort slop and it's far easier to point to the established policy than to explain why the PR is a complete garbage. On Github it's even easier to detect by inspecting the author's contribution history (and if it's private it's an automatic red flag).

Of course, if someone has used LLM during development as a helper tool and done the necessary work of properly reviewing and fixing the generated code, then it can be borderline impossible to detect, but such PRs are much less problematic.