Hey,

some therapists don't really like labels, they see it as a way of avoiding what might be really causing the behaviour and instead of looking for reasons why you behave a certain way, you may just say "it's my bi-polar."

As you said, Therapists don't diagnose, only a psychiatrist can say that you have bi-polar or not. others may be able to suggest or ask you if you feel you have it but that's about it. You are only describing one incident that may suggest a manic episode, but it could also suggest just a boost in confidence or feeling good about being where you are at the time, a diagnosis of any sort couldn't be made from this.

If your therapist was to say " i think you have bi-polar" and then you went to a psychiatrist who said " no you dont" you may end up feeling mad that your therapist told you that you did!

As you said your T is not dismissing the possibility but perhap he hasnt seen any other behaviours that warrant him to think you have bi-polar anyway and that he's not hidding anything from you? You are presuming he is.

When he asked what you would get from a formal diagnosis, I think what he means is, if someone told you tomorrow that you had bi-polar, would it change how you feel, how you act and behave etc? No it wouldn't,you would be the same person you are today only with a label.
I understand how having a diagnosis can at least shed some understanding on the subject but if you really want to be assessed for it then you need to go and see a psyhiatrist.