I agree that two brief meetings with an inpatient psychiatrist is not enough for a major change in diagnosis, unless you clearly exhibit undeniable symptoms. Full blown mania usually fits that category.

I have no idea what your past history with psychiatrists​ was prior to your hospitalization. Did a past psychiatrist witness you clearly hypomanic or full blown manic? And did those episodes fully meet the criteria for such episodes? Time requirements, enough symptoms to meet their criteria, and others?

From what I've read, Borderline Personality Disorder's emotional dysregulation can seem a bit like what some people call rapid cycling (or specifically ultra ultra rapid cycling). In some books and online articles, true ultra ultra rapid cycling bipolar disorder is said to be fairly rare. I've also read that there is the occasional psychiatrist that doesn't even believe in it. [Not my view, because I don't know. Just what I read.] That certainly can be a reason why some psychiatrists doubt that diagnosis in favor of Borderline, if clear Borderline symptoms are also firmly noted. In any case, mental health practioners have their own views about mental illnesses. I do not have Borderline PD, but I remember as part of a conversation one of my past psychologists saying that he didn't even believe BPD exists, despite it being in the current DSM. He was a lousy psychologist, I'll admit.