It's presented as a dismissive question with no answer. But with the number and amount of antibiotics prescribed, wouldn't you expect a lot of curious, seemingly miraculous changes by accident?
You know when you take antibiotics. You have a clear time frame, and the event is unusual enough that it should stand out.
Except that for a cause-effect relationship, you need a kind of "all else being equal" scenario. But since we take antibiotics when we are ill, we already are in an unusual state of the body. So any mood swings, if noticed at all, are just as likely (and given that we tend look among the familiar things first, I'd say more likely) to be attributed to things like poor sleep or other side-effects of whatever disease caused you to take antibiotics.
Well, the people in question are already in the hospital or long term care for some reason, so there's an unfamiliar environment, stresses related to why they're there, etc. Not hard to find reasons for behavior or mood changes there even without addressing the microbiome.
You know when you take antibiotics. You have a clear time frame, and the event is unusual enough that it should stand out.