I can imagine pretty ugly retcons, but how does this one qualify as such? ???
Because a nation state affects geo-, err, stellar politics around itself. It has concentrated forces and economy, unlike scattered independent planets, thus a threat but also potential ally or subject. Its culture may affect nearby planets part of other nations (we've seen that before).
Quite a bit of effects that need to be explained, i think. Like during the Fourth Succession War, the Auriga would be a potential ally for the Federated Suns.
A lot of potential there, hence ugly. Now, move it 50 ly (or more) from the borders and it becomes less influential and important, perhaps, this would not be so ugly.
For something worse: try timeline compression or outright ignoring or forgetting things, or rewriting history but still referencing original events...
Reckon i'll read the sourcebook once it is out and then make judgement about just how ugly retcon this is, who knows, maybe it is done well actually. But for now, it looks ugly at least to my mind.