The way I saw it, the tumor didn't directly destroy the two universes. Both versions of Jack from the trolls' and kids' sessions caused the actual destruction. Meanwhile, Doc Scratch misled everyone by saying the tumor had the power to destroy the Green Sun. While that was true, it also had the power to create it. Which it, in fact, did. The sun wasn't there when Rose and Dave arrived to its location because it didn't exist yet. It was always meant to be created by the tumor