1. Link is depicted as BotW link. While an alternate costume is a definite possibility (ala Hyrule Warriors Definitive), the way that this version is the only one shown might mean that this is how Link is mainly going to be represented in the game.

2. The logo is completely different (as well much more simple and sleek) than the one used for Smash WiiU/3DS and doesn't feature the "for Nintendo Switch" tag. If it was an enhanced port this would most definitely be there. The official Smash Bros. website even refers to the game as "Super Smash Bros. (Title Pending)" and not "Super Smash Bros. for Nintendo Switch".

3. The copyright at the end only mentions Nintendo and HAL Labs. Namco Bandai was the main developer for the 3DS and WiiU Smash titles. If this was a port, they would be there. (This could explain the seemingly shorter development period compared to previous Smash games as a different company could have started earlier.)

4. Just the general mystery surrounding the game, if this was just a port there is no reason to be this mysterious. It would be a more straight forward announcement, like the other WiiU port announcements have been and definitely would have included some gameplay and a basic overview while still being a surprise.

5. Some might say that 4 years isn't enough time to create an entirely new game , especially since Smash Bros. titles are usually 6 years apart. But I disagree. 4 years is still a long time for a development period. For example Batman: Arkham Knight took around 4 years from Batman: Arkham City's release and look how much that game improved on it's predecessor. Or even Ubisoft going from Assassin's Creed Syndicate to Origins in only two years! As always, they have the previous engine to work off of so they're not exactly starting from scratch. And besides, previous gaps in the Smash series' releases have only been long mostly due to the gaps between console releases with Smash WiiU being released farther into the WiiU already short life span.