Timelines are not all set 100%. If they were there would be no free will = superdeterminism.
It's not that Q went back in time as much as Q being a project to make as many people as possible aware of the situation so the timeline that WAS observed (through technology) could be changed. Our collective timeline is chosen by the masses conscious intent, nothing else.
That is why the primary focus of both sides in this here WAR is the opinion of the public, which is the basis of which reality our civilization will experience.