That's not evidence. You want evidence? Look at the trailer.
-Worked on with HAL Lab + dubbed as "original title"
-BOTW Link (this one is debatable, but it at least confirms it isn't a STRAIGHT port)
-New characters - the inklings, also confirms it isn't a straight port
-If this game was a port, we would've seen gameplay
-Sakurai said he was working on the game silently day by day. I don't think a port would require that much effort
The only thing that suggests it isn't a new game is the fact that they're planning to release a NEW smash in 2018. Unless they're going the melee route and are going to complete this game in a very short time, there's just no way they'd have enough time to make a polished Smash game that isn't somewhat reminiscent of Smash 4. We can definitely expect a lot of reused assets in this new title.