It would be an interesting/thought provoking subject if it were based on factual evidence. He stated that BPD was initially classified under Bipolar, which I have not found evidence to support that. In fact, on this website, Borderline Personality Disorder Demystified - history of the disorder , it states:

In 1938, the American psychoanalyst Adolph Stern first described most of the symptoms that are now considered as diagnostic criteria of borderline disorder.5 He suggested the possible causes of the disorder, and what he believed to be the most effective form of psychotherapy for these patients. Finally, he named the disorder by referring to patients with the symptoms he described as “the border line group.”

It goes on to add:

The psychoanalyst Robert Knight, in the 1940s, introduced the concepts of ego psychology into his description of borderline disorder. Ego psychology deals with mental functions that enable us to realistically perceive events, successfully integrate our thoughts and feelings and to develop effective responses to life around us. He suggested that people with borderline disorder have impairments in many of these functions, and he referred to them as “borderline states.” 4

And I will stop here:

The next major advance occurred in 1975 when John Gunderson and Margaret Singer published a widely read article that synthesized the relevant, published information on borderline disorder, and defined its major characteristics. Gunderson then published a specific research instrument to enhance the accurate diagnosis of borderline disorder. This instrument enabled researchers over the world to verify the validity and integrity of borderline disorder. Subsequently, borderline personality disorder first appeared in DSM-III as a bona fide psychiatric diagnosis in 1980. 4

These are only three paragraphs/statements which concern the history of BPD. No where on the site does it mention that BPD was first considered to be a "mood" disorder. In one of the uncited paragraphs, it distinctly labels it as a personality disorder, and was classified as such in the 1940's. Check the link, and notice the endnotes. This is all based on fact, not conjecture.