I concur; the graphs didn't show much difference to me (but I was on my phone so I couldn't be certain :-) The study seems impressive from the title, but reading it not so much. The Alzheimer's patients in the study are older, more male, and less educated than the controls, so it seems quite likely that they are eating worse which would explain the different microbiota. This could mess up the mice in multiple ways. (Seriously, the mice that receive Alzheimer's microbiota could have a stomach ache and do slightly worse on the tests.)

As for why there is so much interest on HN, I've noticed that Alzheimer's is one of the topics that is overly popular on HN, along with category theory, Lisp, and problems with Tesla, to name a few.