I've heard the argument that borderline PD really is just PTSD, but it's PTSD done at an earlier developmental level vs. the usual PTSD we see in adults. I think there is merit to that argument, though it is a bit more complex than that. E.g. SSRIs don't offer as much benefit for borderline vs. PTSD, several issues are going on in borderline other than anxiety, etc.
But it's certainly someting to consider, and the two are often comorbid. I think it's kinda like the bipolar-psychosis spectrum. They are two distinct entities, but there's a lot of overlap.
Is it ego-syn or dystonic? I don't think it's possible to say this for all cases, or even a significant majority. There's a lot of black and white thinking with BPD. It can go either way.
Someone close to me was severely molested as a child. She has PTSD, but does not have borderline PD. Her parents were very supportive, gave her a loving environment, and helped her with the trauma she suffered.
The two disorders go hand-in-hand, but I still see them as separate.