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.
Yes, the room designs come from assets seen in the actual Tomodachi game. However, I think the Tomodachi stage is still as close to a smoking gun as possible -- and really makes this rumor all but confirmed:
1. The room in the leak and the scan are identical to the T. Same color scheme, same objects and same object placement. That's one insane coincidence.
2. It assumes the leaker went out of his way to fake this one extremely minor detail -- yet did not put the same effort into the rest of them.
3. Even if he wanted to fake it, how would he do it to make it so exact? The camera angle and graphics in the Tomodachi game aren't exactly the same in Smash. The graphics are close, but the angle is different (and also exactly the same in both the scan and leak).
4. The leak came out before the Famitsu scan, so there was no reference material for this layout.
I know people want to be cautious, but I think Leaker 2 is on the money. Unfortunately, Leaker 1 is the one that did the character select screen. So, characters and the DLC are still up in the air -- except for Shulk who is in both leaks.