Second, while the author may not understand the code at a level where they could have written it themselves, they should still understand it as a reviewer. Treat it like the agent is the author and the human author is the first reviewer; don't pull somebody else in until you're satisfied first.

Emphasis mine.

That brings you back to square one. You should only be satisfied with the state of the PR once you understand it. Anything below that and you’re not doing your job properly. That doesn’t mean the code needs to be flawless on first PR, but you should understand everything it is doing and the why of the approach, otherwise there’s no point to you doing it.

So to the question in the title, the answer is quite simply “never”.