Well it is appearing in the manga so can you really call it a filler. At least in the terms of anime filler is a period of time in the series that is not canon. Since it is appearing in the manga it is canon. It is not like Tite has not gone and done something random in the middle of his arcs before. There were two pit stops the made back to the real world during the Soul Society arc.

Though what has me puzzled is how can be a hundred years ago and Urahara and Yoruichi are still around. Last I checked they left the Soul Society a hundred years ago. I know that Soifon made mention that it has been a hundred years since Yoruichi has been gone. So that does not make any sense.