(Edit: Sorry, as you can see I have never had the talent for conciseness.)
The first reference in the texts to jacking on her back doesn't make sense as a taunt. Both texts are about how angry he is that she's trying to extort him, angrily talking about how she and her mom should pick an easier mark next time, and so on.
The first part of the text to me read like, "We were intimate, we did all these things, I can't believe you're extorting me." For example, an analogous text might read, "We walked along the river holding hands! We had romantic dinners by candlelight! We were sleeping together! How could you sleep with my best friend and say we were just friends?"
To read the first part as a taunt doesn't jive with the second sentence, "slept with u n bed!" AB is saying "we were really intimate. We slept together in bed. I jacked off on your back."
The reference to "jacking off on her back" could easily just mean they were doing doggy and he unloaded on her back at the end instead of inside her.
Now look at the second reference to jacking off on her back in the second text. me n (redacted) a laugh about this **** me busing on your back!" He calls it "this ****." Let's say he did jack off on her back, but only in the context of finishing after doggy style. So to AB, the fact she's making up some ridiculous story about him masturbating and finishing on her while watching a religious video sets him off. That's why he calls it "this ****" and the story's so stupid that he and his friend(s) are laughing about it.
Now to me that's an easily plausible explanation. Both stories are going to be unprovable.
Regardless, the original interpretation doesn't make sense to me, because he's clearly saying, "we slept together in bed, we were intimate, I can't believe you're coming up with this extortion garbage and texting my team, etc." Wouldn't follow logically to frame an argument, "(<TAUNT>) (<EVIDENCE OF INTIMACY>, <HOW COULD YOU DO THIS>"
haha I pulled the chair out from under you when you were sitting down! We were intimate in bed, so I can't believe you're pulling all this!"
Instead, to me he's clearly doing,
EXAMPLE OF INTIMACY (jacking off on her back after doggy)
EXAMPLE OF INTIMACY (we slept together in bed)
then following with, "WE WERE REALLY INTIMATE, HOW CAN YOU BE TREATING ME LIKE THIS."
