Maybe we can tie some things together here.
You're curious about some of his behavior and choices, and Cat Familiar has sort of "jumped to the last chapter of the book" in asking whether that's what
you want in your life. (Spoiler alert, that's where I might end up too, question-wise!)
That being said, we can juggle a few questions/themes at once.
A really specific answer to your questions of
Why would he talk to me that way?
and
My thing is, why ask for phone sex? Knowing we aren't together.
would be, again, that this is the disorder showing up loud and clear.
pwBPD struggle with boundaries and with intimate relationships. With less-close people (new acquaintances, coworkers), some pwBPD come across as "mostly normal" or even fun, charismatic, deep thinkers, etc etc. However, as the relationship gets (or got, in the past) closer, typically more and more of the dysfunction and disorder becomes apparent.
For a "broadly normal" person after a breakup, it would be "obvious" that you don't have a polite text conversation and then bring up phone sex.
What you've just experienced is the disorder. It won't make sense because to "broadly normal" minds it doesn't make sense. This isn't a normal thing to do.
In the past, maybe you've gone with it, for various reasons. Now, you've decided for yourself that you have a boundary for your life: no phone sex. pwBPD may "talk a good game" about boundaries, or not, but either way, struggle to understand and respect boundaries from other people. Lacking an internal sense of self, and unable to really differentiate what they feel from what others may feel, they may have a tacit belief that "everyone, especially people I'm close to, should feel like me". It's incomprehensible and infuriating to them that others would have different needs, feelings, and boundaries. If others do, then
they lose their sense of self, because it's externally derived.
I wouldn't necessarily label what he's doing as a tactic, as that perhaps gives it a level of planning and thought that it might not have.
He certainly seems to want something, and in the past, the way you responded "trained" him (sorry, best analogy I can think of) that him saying that would work to get him what he wants.
Now that you are making different choices for yourself, he still wants *whatever* from you, but his old way isn't working.
...
So, all that being said, a couple of things come to mind:
Everyone who knows me, knows that's just how I talk. Not just to him, to anyone.
Now that you know he is disordered -- that his brain doesn't work like a normal brain -- you have some choices to make about how you communicate with him. I get where you're coming from, that "normal talking" works for most of the people in your life. As you're discovering, "the way we typically communicate" doesn't work well with pwBPD. So you have an opportunity to decide if you want to try different communication with him. Continuing to communicate with him as if he had "broadly normal" mental processes may not give you results you want.
if he starts a conversation that would make him look stupid, in his eyes. If I start one then he can continue to be in control of the situation and stand his ground with the ultimatum.
Do
you want to keep the conversation going? He's going to think whatever he thinks (and the rationale is probably weird and not very understandable). The good thing is that you "drive the car" on whether or not to keep talking, on your end. It's a decision only you can make, and this is a good forum for working through what you'd hope for, if you choose to keep texting/chatting/etc. You also have the option to not stay in conversations if you don't want to (for whatever reason) or to end a conversation at any time, and not pick up "bait" from him.
I get that there's a lot (i.e. everything they do

) that we can't control from a pwBPD. Again, though, the good thing we learn here is how much we do have control over our level of involvement.
As more members get involved in your post and offer feedback, it's pretty likely that you'll hear from the "been there done that" crowd that we can experience more success (or at least make things "less worse") when we pivot our focus from "why is he doing that, why did she say that, how can I make him understand X, why can't I make her do Y" to...
what do I want
how do I want to be treated
what is working for me
what isn't working for me
how can I change what I do
As counterintuitive as it may seem, focusing on ourselves, our wants, our needs, our motivations, etc, and using our limited energy to understand us, versus disordered behavior, can actually be more productive.
Lots to chew on...
kells76