In my experience, what Millon says is fairly accurate. HPD and BPD are definitely similar and share some of the same characteristics. I also agree they can be quite tricky to tell apart, depending on how symptoms and characteristics present. The only part of that quote that I haven't seen significantly first hand is the part where he said, "developmentally, histrionics enjoy a special relationship with their opposite-sex parent that stops short of actual incest ..." Individuals with HPD often do sexualize relationships and I have seen this, but the way it reads it sounds like this type of sexualization towards the parent is a very common occurrence (as if they all do it towards a parent) and I can't speak for that based on my clinical experience so far.
One of the above posters (i can't see it on the page right now) mentioned how some PDs, particularly ones of the same cluster, can look similar and how it's important to look at the root cause of the presentation/behaviors/symptoms. I think this is the trickiest part about the PDs. For example, some patients with BPD can come off VERY narcissistic, but sometimes when you strip the behavior down, the "activating ingredient" of the behavior isn't really about a grandiose sense of self at all. But, of course, it is completely possible to have a diagnosis of "Axis II: BPD with narcissistic features" it just depends on the person. (am I making sense? it's like 330 am here.)