That is probably a mock up, but also probably based on the actual design of it. I did the math, and it could fit 52 characters on there.so there is room for 3 more characters then the 3DS screen. If it's just a mock up they could possibly fit even more, like resizing and such.
Now, this was posted on another thread here
It LOOKS like it could say tower of smash there. Also, the leaker said tower of smash is a solo mode, so it makes sense that it would be in solo and not where smash run is. Smash run is not solo, and I think whatever mode they may have in smash runs place would be multipler.
That is IF the menu's look exactly the same.
So I think the leaks are still alive. Both of them