I think, it might be a case of timelines existing independently in various spaces and dimesions. Time goes relatively depending on observer (while it was outright stated that each Universe has its own timeline, judging by Jade interacting with Jake, even within one universe separate timelines can exist (with given definition of separate since alpha came from reset of beta) and until you do come within a timeline, it is like a story to you, interactive mayby, but you are not "an actor" just yet.

It’s really a curious conundrum Homestuck poses, and the specifics of Timeline Shenanigans are one of the few basic things Homestuck does I’m not 100% certain about quite yet. There’s so many variables.