User blog comment:FairyRaven/Fairy Tail Chapter 329: Seven Dragons, Review and Commentary/@comment-49.176.34.76-20130430152649/@comment-7418318-20130430154932

Sure is really is a filler, in the technical sense of being created to avoid that the anime catches up with the manga.

But that doesn't mean that the story of this filler arc can't be taken over into the manga, if the author wants to (barring possible copyright or other legal hurdles I suppose).

And for reasons already mentioned, it seems that Mashima did indeed want to do just that.