Here's reasoning in that case.
-It's boxart that can be changed
-Sakurai would not let this slip past his radar
-The boxart would be covered up by now just like how Ganon and Lucario got leaked.
-It is boxart
the boxart of the ultimate edition ( the black big box with gamecube controller) was changed for onlinestores every time the banner was changed, so before you could not see k rool, simon and co, but now they are on there, it could totaly be that we also have the back of the box also using character slots, i mean technically you could take the original 8 on the back cause you start with them, and the others on the side opening up 8 more spaces THEY ARE NOT ON THE SIDE
and then you hav the others on the other side, this makes sense for advertisement if you see the box with the context,
you start with smash bros legendary 64 base roster, for speculation this makes sense cause that means like 10 or so still are missing right?
1 on each side? and then additionally 8 cause of missing allstars on the back,
so we can expect still 4-5 more newcomers and 5-6 echos which makes 10000% sense to me