You should be rejecting the PR because the fix was insufficient, not because it was AI agent written. Bad code is bad code regardless of the source. I think the fixation on how the code was generated is not productive.
If an actual developer wrote this code and submitted it willingly, it would either constitute malice, an attempt to sabotage the codebase or inject a trojan, or stupidity, for failing to understand the purpose of the error message. With an LLM we mostly have stupidity. Flagging it as such reveals the source of the stupidity, as LLMs do not actually understand anything.
I mean they probly could've articulated it your way, but I think that's basically what they did... they point out the insufficient "fix" later, but the root cause of the "fix" was blind trust in AI output, so that's the part of the story they lead with.