because her story when asking for love advice was 1:1 exactly what just happened to him.
OK, I completely forgot about that part and I can see a line connecting the two dot eventually, but it's quite far reaching. I can see it serves as a point for MC to reach the conclusion later, but since he hasn't mentioned anything about that yet, I still don't quite want to make the assumption MC already realized it.
To me, MC should just be under the impression that this girl he knows IRL is smurfing for some reason and only suspecting based on her skill, voice, and dialect that Ayaya would fit the bills for that, but there's no other clear evidence; only affirmation is that a rando also suspecting the same ...probably because MC is outed as Oni-chan so he might suspect the girl with similar skill, voice, accent, and has recently collab with Oni-chan might be her. Base on that, if she were to insist she isn't Ayaya, though hard to believe, I can't see anyone proving otherwise, until she said she was trying to hide it and tried to cover it up.