Something had to be said. I'm not totally convinced that was the best such something. Although maybe we'll get to something useful now the log jam's been exploded.
The key for me is when she saw that over a week, everyone (particularly Mittsu) had not failed to improve, but gotten significantly worse. What's killing him isn't the lack of time, it's in his head--he's second-guessing every note, so he sucks. And everyone's "making allowances", so they start to suck too.
As to the guy who said it, I'm waiting for him to hit what I'd call the "prodigy wall". He's very skilled and doesn't need to practice much to do the song right. He can put out a very competent performance. But what happens when what's needed goes beyond competence? How does he produce a unique interpretation, a sound with depth and emotion, when he's shallow and avoids contact with his emotions? He's going to need to dig deeper at some point.