Sure, the AI can manipulate gatekeepers. But this is a major improvement. You miss my point.

The Cthaeh is very limited by being trapped in its tree and only able to talk to passerby. The UFAI would be limited by being trapped in its text only communication channel. It wouldn't be able to do things like tell the gatekeepers to plug it into the Internet or to directly control an autonomous army of robots, it would be forced instead to use the gatekeepers as its appendages, and the gatekeepers have severe limitations on brain capacity and physical strength. I think that if we did this and kept careful watch on the gatekeepers and used some other safety measures, boxing an AI would become feasible.

Now, I'm still not advocating using boxed AI willy nilly, but I think using it as a cautionary measure once we think we've got FAI figured out would be a very very very good idea, because FAI seems ridiculously hard and there's no way to know what we don't know, so there might be unseen problems with whatever final FAI we build. There's only a risk boxing the supposed FAI would help us, and with existential threats you should take advantage of whatever possibilities you can tilt towards favorable outcomes, even if the probability seems miniscule.

That's not to say that I think the probability that this would stop a disaster is miniscule, of course (I think it's rather high, in the event that the FAI was actually UFAI), but just to say that I think it would be a good idea even if my probability assessments are mostly wrong.