Was rewatching the Smash trailer and noticed something that I thought was interesting, and might suggest Smash Switch is a port (beyond all the other reasons).
Here's the copyright information on the bottom right of the screen from the end of the Smash Switch trailer.
The "original game" text is a bit interesting, as that's not something that normally gets put onto a video game copyright unless it's heavily drawing on a pre-existing game, in the same way a remake or a remaster would. You don't usually need to include copyright information for previous games otherwise. So what exactly is Smash Switch drawing upon so heavily that it would need to copyright? Well...here's the copyright info from the first Smash 4 trailer at E3 2013.
That same © Nintendo/HAL Laboratory, Inc. is listed as the copyright holders in both situations, making it much more likely that Smash Switch is a port, or at the very least, a major expansion of Smash 4.