I think had you tried to pin your daughter down in the same way and force her to take some accountability for her behavior, she'd storm out of the conversation like that as well. So it's not that their minds can't focus: the rants, the pointless circular arguments, the finger pointing, deflection, etc. is all a game they knowingly play to avoid having to accept any accountability for their behavior. Because if they do that, then they need to change their behavior, and they'd sooner die.
In my daughter's case, what she was saying/doing was hurting her far more than it was hurting me. The more she rambled, the more distraught she became and there was actually very little about me in all her talking. It was just a jumbled mess.
In your situation, I can understand how someone thinking in a disordered way would try blame-shifting to avoid talking about the larger issue. You and I both know the real problem- it was disordered thinking and not being able to control responses.
I had a similar incident with my BPD ex wife. After we were separated for 3-4 months, she asked me to meet her at Olive Garden after church to talk. I thought, okay...progress. Maybe something is starting to change. But once we were there, we talked a bit and when I told her that I still thought about her daily, she began to scream at me in the restaurant at the top of her lung. People were startled and stared, my wife was embarrassed. But anytime I said anything about us, she'd lose it all over again and the whole restaurant would freeze in shock. I'm surprised that we weren't kicked out, to be honest.
My ex wasn't doing that for dramatic effect, she literally couldn't help herself when she heard me say something that her disordered mind didn't agree with.
Later that day, my wife told me in the car that she had feelings for another man and wanted to pursue that relationship. It was just a few short sentences amidst hours of disordered dialogue, and I realize now that it's the only reason she met with me that day. That's why she didn't storm off after screaming in the restaurant, and why she asked me to take her shopping afterwards. It was all for that singular goal, but it took her almost three hours to get there. The "calculated" part was two sentences; the disordered part was three hours of nonsense that prevented her from saying those two sentences.
For your situation with the therapist, I can't say how much was calculated and how much was just emotion flowing. Maybe we can't know for sure. But when I look back at the worst moments with my BPD ex and my BPD kid, their pain was real in the moment and they weren't benefiting from it. Other people also saw what only the closest typically see...the true nature of BPD and how ugly it makes them in those moments. I don't think that's intentional either, to let others realize how messed up they are.
That's just my opinion though from closely observing two people who displayed BPD in completely different ways. My wife was the quiet type while my daughter was traditional explosive anger all the time.