Prebiotics and probiotics are not the same thing.
Prebiotics are kinds of fiber that probiotics can use to feed off of.
Also you have to be careful about reading study headlines - like that one that omega 3/DHA was “useless” except they only looked at heart disease and there’s plenty of evidence for other disorders.
As for probiotic pills, yeah, mostly you won’t significantly change your intestinal flora, but a high fiber diet rich in prebiotics can help.
I take a prebiotic called Bimuno.
I linked a blog post from some autism blogger recently on how it was largely pro and pre biotic changes that offered neurological benefit in the ketogenic diet.
Edit: here it is How ketosis and gut bacteria change levels of GABA in the hippocampus