I think there are a several reasons to believe it's a new game.
- The version of Link shown was based on Breath of the Wild. Admittedly that could be a new skin if it's a port, but that seems less likely to me.
- HAL Laboratory was credited for it, not Bandai-Namco.
- They showed us very little. If it were a port, I would have expected them to show more stuff we recognized from Smash 4, and make the big deal that it was coming to Switch with extra content. Instead, we were shown three characters. That's the kind of thing they more typically do for a new game. It builds hype, and lets them keep the hype going by showing us just a little bit more every now and then.
- It has a dedicated website. Seems like an odd thing to do for a port.