Twila died at the hands of Buffy but was then resurrected into a timeline where Buffy was much dumber
It is the only hypothesis I can believe to explain why Buffy is so fucking stupid here. Why did none of the people that were in the pact with Hamez die after Jacob died in a car accident, even though Hamez said that if any of them died, all of them died? Well...
HE LIED. He only said that to avoid getting killed by Jacob. In fact, the pact may actually work in a much different way. Buffy had an off-screen battle with Twila and Edward
When Buffy comes to kill Twila and Edward, Twila threatens to post pics of her fucking with Angel on the internet, yet after the narration says that she got scared because she didn't want that, it simply says that "things happened and then Edward went home". The most in-character thing that Buffy would do in such a situation would be try to kill the vampires despite the threats (you can't go through with a threat like that if you're dead), which would have ended up in Twila and Edward fighting the vampire slayer, normally this would've ended in the two of them getting killed to death by Buffy, but since Twila is such a fucking Mary Sue, she likely managed to defeat Buffy and made her flee the house.