In the leak's Tomodachi stage slot, there's a previously unseen room. The exact room was seen again later on in the recent Famitsu scans.
They have shown a new stage in famitsu yesterday which already is on the leak. That is the reason that I believe this leak cause there is no way that they knew beforehand and also had a pic of the stage already.
I haven't played Tomadachi, but the Tomadachi stage gets the rooms from designs randomly from the actual game, so it might be possible it is one hell of a coincidence.
I still don't think this Leak will be "confirmed" or "deconfirmed" until the next character either veteran or newcomer is revealed.