No he didn't. Mach Punch is a variant of Quick Attack, so Infernape would strike first.
However, he did miss one thing: What if Mewtwo also had a Focus Sash?
That's why you use fakeout. But I was thinking of taking out tithe Mach punch as I knew it'd be a little added damage and people would laugh at it, I'm pretty sure a stab+blaze overheat would be enough to ohko it unless it had pumped up SD. --- my sig was getting old.