All things considered, that comment was about as tactful and polite as can be expected under the circumstances.
He's going to have hard time moving on, and odds are he will never be able to truly escape the shadow of being the "ex-DT drummer". Even if Adrenaline Mob or one of his other projects becomes more successful then DT, he will always be in a position of having to live up to (and, perhaps sometimes, live down) his reputation of having been the drummer of Dream Theater for 25 years. Ozzy was never able to fully escape his history of being the original singer of Black Sabbath, and his solo career, by and large, was a much greater success (commercially speaking anyways. If you want to debate that on an artistic level, then you have an open argument).
Ultimately, I think what will be the most difficult thing for him to live with is just the knowledge that there actually was a Dream Theater after Mike Portnoy. I'm sure he will be constantly second guessing his decision to quit for quite a while before he finally comes to terms with his new career and wherever that leads. Every time he talks about any of his upcoming projects, he seems refreshed and recharged, with a new found sense of purpose, in ways that he hasn't for a long time in Dream Theater. I'm happy that he at least seems to have found his muse again, and that he actually feels motivated with his new material. I guess the pain of not being in DT is the price he has to pay for that artistic freedom. Was it worth it? Probably, but it may take a long time before he can fully appreciate that fact.