Basically Tsumugi called herself a copycat criminal offhand at one point during the last trial and Shuichi remarks at the very end that the phrase suggests that Danganronpa wasn't just fiction and it was Tsumugi's version of events that were basically the lie because she wanted to be the new Junko.
So when Danganronpa 4 happens they can just say Tsumugi was a mad bitch and Hope's Peak and everything else actually happened.