No, we are at peak “gut bacteria” science denial. On the other hand study of the microbiome is still in its infancy and looking very promising vis-a-vis human health.
At the risk of stating the obvious. Gut bacteria are bacteria... in the gut. Like if you tell me they affect the absorption of nutrients - I can see that. Maybe they make something more bioavailable or maybe they break something else down. If you tell me their involved in deficiencies and over/underweight that kinda makes sense.
Tell me they affect appetite through manipulating our signals ok. Not as obvious but not implausible either.
Tell me they cause highly complex behavioral differences... it just seems implausible. But maybe gaba can indeed explain it.
Nowadays, the gut is sometimes referred to as the "second brain" because of your enteric nervous system which is located inside the gut. It probably has more influence over you than you're aware of, as it operates independently of your CNS.
If you're talking about the time before there was strong evidence for pathogenic bacteria- then that would have been the only rational and justified behaviour.
Why should anyone believe something that sounds implausible given current knowledge, before strong evidence is presented to support it?