The concept of worldlines is not exclusively a KHUχ thing, a lot of stuff with worldlines happened at the end of KH3. It's basically KH's way of saying "timelines." The KH3 Secret Reports talk about it a bit.
At the end of KH3, when Sora changed the events that happened to them at the Keyblade Graveyard, instead of creating a separate worldline/timeline like you think would happen when something in time was changed, Sora straight up broke the laws of nature and rewrote history on their own worldline. So, instead of there being two worldlines, one where light expired and one where light prevailed, both of those histories exist on a single worldline. The worldline goes up to the part when light expires, and then there's a tear in the worldline, which is followed by the events of light prevailing.