I've done some research into the development time of past smash games.
Brawl was announced at e3 2005. At that point sakurai was not involved with the game. Development started around October 2005 and the game released in January 2008. Just a little over 2 years development.
Smash bros wiiu/3ds didn't start development until after kid icarus uprising which released March 2012. Those games released in September and November 2014 giving those 2 games about 2 1/2 years development.
I'm not saying for a certainty that this is a new smash. But saying that they haven't had enough time to develop it just isn't true. If development started around 2016 that is in line with the development time of the past two games.