TBF, Miku has a great ass for her size too.Waaaaay too flat to be her ass, even Miku has more rump plump than that, that's 100% dude ass right there.
Oh also apparently Sel confirmed today that Kaori is NOT Rin's mom, so if you care about 'word of god' stuff, there's that. or you can choose to believe he is playing unreliable narrator in the server for your theories