Transmasc

As an observation, I think it's valid. As a criticism of any kind, it's shit.

I refer to my man boobs, sure, but don't really say breasts. It is what it is, anecdotally of course.

But I also don't know I'd even notice if a friend of mine said something about his breast.

Also not trans, just want to be part of the conversation.

Or maybe more directly, referring to breasts as "breasts" does not in an absolute sense disqualify someone from being a man.

Even if this seems trivially true, it doesn't stop people from taking generalizations (like men typically say chest rather than breast) and applying them in either a policing fashion or in a disqualifying / gatekeeping fashion.

OP might just need to talk to the therapist about the comment and walk through how they experienced it.

In voice therapy a trans person might intentionally train to mimic common stereotypes in gendered speech patterns, sometimes it's just pragmatic and useful to be clued into those gendered generalizations to help pass - we don't know if the therapist was trying to likewise be helpful and accidentally came across as policing or gatekeeping.

Wishing you luck OP!