The emphasis would be incorrect if the term were reversed - RankBrain is a Brain for Ranking like PageRank is a Rank for Pages.
No doubt there is someone in Google, however, who has a project for ranking AIs called BrainRank for their own amusement.
Anyway, your observation is also an amusing interpretation.
PageRank is called after its inventor Larry Page.
It's just another case of Nominitive Determinism!
Amusingly enough one of the definitions of "rank" is, according to Wiktionary, "having a very strong and bad taste or odor"... as in the smell of a decaying brain. How fitting. I almost wonder if it's deliberate. If they called it BrainRank (like PageRank), the adjectival meaning seems to be emphasised less.