If that were the case, he would've slaughtered Gengetsu. But he didn't, he could only pull a stalemate.
And Gengetsu was beatable by Gaara and no-Jinton Onoki, so Mū isn't as strong as some would hype him on here.
Hanzou took the sannin and still coulda won, the dude was feared, but in the war he did'nt do jack, and gengetsu is not a good matchup for muu, in order to find his clam one also needs to be a contact sensor well not a bad matchup kinda like their abilities are hard to counter, and jouki boyis a pain in the ass to deal with, not to mention kishi did'nt want to waste time with the edo army so he pitched them with opponents that counter them easily, and it took gaara, ohnoki and kcm naruto to seal muu (though he was still lurking) apart from madara and nagato he was the third edo who was stopped by strong opponents