I think the key here is that the extra was run in a different magazine(both Winter 2008, no less!), rather than being newly publishes in the tankoubon. Since it's a one-shot sequel to a one-shot that was published at about the same time, I don't think Sayuri-hime's editors would've accepted it without an independent plot. That said, I think in a way it's the perfect sequel: what we see is basically telling us that the happy ending of the main story was a happily ever after.
