I've a few things to say about this, I don't know if they'll be particularly good or relevant. First of all that's not just any professor, that's Kirill Gerstein who is an incredibly respected artist with a major career. His playing represents some of the highest standards in any pianist anywhere, so this was always going to be an intense lesson.
Sean's playing is what you'd expect from a talented conservatoire student. It isn't the best out there, but it's by no means the worst. It's a little dry sounding, and he's making that rookie error of putting a lot of intensity into the performance which isn't translating through the piano. It's OK though, this happens a lot and it's nothing to be ashamed of.
Kirill Gerstein will look for students who can win competitions, and Sean Kim is not that student really. Or at least not yet. I don't for one minute mean he won't be able to become a concert artist, and he may even achieve his ambition of having a higher level career than music education, but it's going to be a long hard road for him. Again, that's OK. There's nothing wrong with it. There's nothing wrong either if he 'just' goes into music education or becomes a piano professor at a Community College or small university but is paid a good salary. That's a good outcome. He may yet achieve more than this, and I wish him well.
Sean is good enough to get his DMA in Pedagogy and apply for a 70k a year teaching post in a pedagogy department. I know people who can't play as well as he does who have that job. I've often wondered if playing well might be a barrier to getting such a job though.
Sean himself said it's hard to find a mentor. That means that whatever he was hoping to get from his college experience hasn't worked out for him. That's not necessarily the fault of his teacher but more the fault of the system. His technique is a bit off at the moment, it's a bit underdeveloped. It looks to me like he's had teachers pile repertoire on to an underdeveloped technique which is a common problem in University and even Conservatoire teaching. Sort out the fundamentals and keep working on the fundamentals. Never lose sight of the fundamentals. Every technique problem can be solved with good fundamentals. There are some basics there that he's missing, unfortunately. Note the collapsing of the knuckle particularly in the left hand, and lack of support at the fingertips. MUCH of his technique and pianism is good, but the lack of fundamentals is what makes his playing sound too general. Again, that's OK. This is all OK. He needs to remember he is young,.
The other thing is, whatever he doesn't know doesn't seem to be his fault. He's flying out to Brisbane to play to Gerstein, he's probably got drive. Nothing Gerstein said was bad. Nothing was insulting, nothing was wrong. Everything was actually right. I think there's an element that Sean's hopes didn't match up with the reality. I wonder if he hoped Gerstein would say "you're the student I never had" or something and fly him off to Berlin. Well, the competition is just too tough out there. It's hard to be that student. Sean needs to understand the reality of that, and it seemed to dawn on him a bit in that video.
I do think Sean can learn, I think he'll manage to learn a lot. One thing he's learned is taking a trip for a lesson with a master teacher a week before a recital probably isn't a good idea. The other thing he should learn is that Gerstein has said he doesn't want to teach Sean, but he's said it in a very round about way. It's clear to me that this is a non-starter in that respect. However, Gerstein did recommend other teachers. That could mean that Gerstein knows these teachers, knows what Sean needs, and is recommending the teachers for that reason.
It's a really tough profession, because there are thousands and thousands of piano students who all play to a high level. Very high. Sean needs to work out what's going to set him apart from other students. I don't think it's going to be competition wins. Actually I'd like to see competitions become less important now. I would recommend that Sean finds a technique specialist in person, and finds someone else to mentor him on how to play the repertoire.
Having said all that I've said, I think Sean has a lot of potential if he gets all these things sorted out. Gerstein was never going to fawn over him, and if you seek out a teacher at the highest level then you have to expect criticism of the highest level. This isn't someone who's going to a lesson to feel encouraged and coddled about the recital, this is Olympic athlete level training.
One final note, a master teacher like Gerstein does not take students who will have any kind of technique issues needing to be fixed. It takes quite a bit of time to fix issues. First you have to identify the issue, which is hard. Second you have to convey that to the student, which is hard. Third the student has to be receptive to what you're saying and usually there's a bit of back and forth while you explain and the student hears. Finally, the student has to incorporate the technique into their body which takes time. At master's level a teacher has a year or two to coach the student, get them into the competitions, and keep the massive pay check coming in from the university. They're not interested in resetting technique. There are teachers who can reset technique and are interested in that work. For that kind of work the best thing a student can do is not constantly play recitals and competitions through their course of study.
I feel sorry for Sean, but he'll get over it and he'll get back on the proverbial horse. And if he finds happiness doing something else other than piano, then that's fine too. I wish him well. Nothing I've said here is said with the intent of putting down the young student. All of us who have studied piano have been through this, some of us several times, some of us only once. Gerstein was actually one of the more kind master teachers I've heard. Believe me I've heard some really destructive teachers in the past both in lessons I've played in and attended.