The final phase on Mythic is special, its description in the journal is this:
Do you think we're getting faked out? That the conclusion to Normal isn't the truth and that N'zoth prevailed and we're living in a vision until Mythic N'zoth is defeated and the truth comes out?As N'Zoth the Corruptor's tentacles tighten on the Chamber of Heart, Magni and MOTHER begin unleashing the fury of the Forge of Origination to put an end to the Corruptor's plans.
Or, so they hope.
Personally, I don't think that's the case but it would be neato and I want to believe
It'd be a fun use of the game's mechanics and story, which is a focus of this patch.