I mean its probably a main story but its just short!The beginning of the trailer makes it out to be more than just a side story, IMO.
It is a full project with a lot of animation manpower behind it! so its not something while youre waiting BUT still the wait for Z ish dragonball will probably not be too long!If this is the case and this is just a "here is something while you are waiting" then I will be fine with that.