Has it been established that he is actually angry?
I think it's at least been confirmed he was upset at the time of the aborted trade. Mazzulla talked about a "healing process" at the Jaylen Brown presser (this is also the appearance where he stated White would be starting without any explanation over how they came to that decision). Gary Washburn literally said "he's angry with the team," though he admittedly doesn't know if that's still presently the case.
Ramona Shelburne stated on ESPN "I think a lot of this has to do with what's going on with his elbow." On his Celtics Beat podcast, co-host Adam Kaufman said "I have heard some whispers that they are still trying to trade him," even if both hosts acknowledged that they honestly don't know if Brogdon is still upset currently.
Then you have the fact that almost everyone of note has reported early to camp but Brogdon is conspicuously missing. Now, I'm sure if no trade happens he'll report on the last possible day and say all the right things, because he's a professional. However, the fact that there's been nothing but radio silence from him and the team suggests to me that Brogdon and the organization still have some friction.
On the "this is nothing" side, they've only been adding wings like Svi and Stevens recently. If they were worried about guard depth, either because Brogdon is upset or due to his injury status, you'd think they'd at least add another guard.