I stay in touch with my former psychiatrist in another city. When I was eventually diagnosed with BPII, her response was, "yes, your depressions were more like depressions in bipolar." I read that as saying that the depressions in bp are qualitatively different from those in MDD. A therapist friend of mine said that she suspected that I was bp because my depressions were so deep.
Idk if any of that means anything other than the random thoughts of two mental health professionals. But I thought I'd throw it out there.