They showed the Sacred Timeline as a single line from an outside perspective. Then, after killing He Who Remains/Immortus, it branched explosively all along its length. That reads pretty clearly as "the whole multiverse just ceased to be a single line". I don't see any way to interpret it as *not* being retroactive, especially since they proceeded to show, from a point "inside" the universe ( or at least more inside than He Who Remains abode, anyway ) being retroactively changed.
They didn't just create one new variant timeline, they ended the force that was keeping there from being infinite variant timelines, at all points on the time stream. Because, note, the earlier episodes established that timeline branching was a thing that was *always* happening, everywhere and everywhen, hence why the TVA had to actively hunt down variants and prune divergent timelines. The universe "wanted* to be an infinite multiverse, it was only by the extreme effort of He Of The Bajillion Names and his apparatuses that it could be forced to be just one timeline.
