Granted I am not caught up on Season 6, but I have a hard time wrapping my head around the idea of a mini-series within a series. Couldn't you describe any given multi-episode arc as such?
Of course. But Adventure Time has never had a consistent arc that lasted more than two episodes. And for episodes that continue narratives from other episodes, it's usually broken up by episodes that have little to do with either. They're presenting this "mini-series" as a consistent arc that will carry over for several episodes, hence the distinction.
And maybe later they'll edit and repackage it as a stand-alone movie on DVD or something.