If I recall correctly, Smash 4 took around 2.5 years to develop, at least from when we first knew about it coming. Could have at least begun preliminary stages before then. The final DLC for Smash 4 (Bayonetta and co) was released in February of 2016. It's fair to assume it was finished up a while before then and put through testing phases before being released. The new game could have began development right after that, but more likely could have been in early development while the final Smash 4 DLC was in the works. I do believe not long after the final DLC came out, Sakurai confirmed he was already getting to work on another game, but I don't remember when that happened, or if it was even referring to Smash Switch.
Nonetheless, at the very least, Smash Switch has been in development for around 2 years already, but it's quite likely it's around 2.5 years, and we still have all of 2018 for them to work on it. I doubt a port job of Smash 4 would need to take that long to develop, whereas a full-on new game obviously would eat up all that development time. So just add that factoid onto the pile of evidence we've already seen that suggests this is Smash 5 and not 4.5.
Incidentally, I don't believe Sakurai has confirmed yet that this is indeed Smash 5, he's just confirmed that he's involved with the game's development in some way. Obviously he's gonna play vague with us. He's Sakurai. Onward to E3.
As an added bonus, thanks to the trailer, you can expect these characters to all be in the game: Inkling, Mario, Link (Champion Edition), Yoshi, Pit, Donkey Kong, Diddy Kong, Peach, Ness, Marth, Luigi, Sheik, Pikachu, Samus, Kirby, Zelda, Bowser, Fox, Captain Falcon, and Wario. I'm sure this list is absolutely shocking.