What many people not fully grasp, at first sight, is that offline support for a collaborative editor is at least an order of magnitude more complex than just building a collaborative editor. In online collaborative environments, solving conflicts is relatively simple, as the changes are supposed to be small. But when two (or more) people are working on the same document without synching it for hours or days, the merge strategy has to be extremely good to come up with anything remotely intended (assuming the authors had a common intention).
So just because your favorite editor can be used offline and it has an (online) collaboration feature, does not mean it is capable of being an offline-capable collaborative editor.
So just because your favorite editor can be used offline and it has an (online) collaboration feature, does not mean it is capable of being an offline-capable collaborative editor.