Again, what evidence do you have that it never happened? Homestuck takes a lot of cues from many worlds theory, which states that when there is any probability of two outcomes happening, they both do, creating separate timelines. It's presented as a very real possibility that Terezi may not choose to stab Vriska, which would imply that there is a timeline in which that happens.

It’s presented as a timeline Terezi sees might occur, there’s no real evidence outside of that saying it didn’t truly happen but there also none supporting that it did.