Which leds me to believe 2 things: either they are VERY tight lips about leaks and content or there´s a chance for a delay and they are working nonstop till they really manage to get to that release date. I´m crossing fingers is just nintendo being tight af.
If Melee taught us anything is that if Nintendo proposes a date, they WILL do whatever they must do to get it out on time. But I doubt that is the case; it is believed that Ultimate has been in the plans since 2015, when the last DLC was announced and released. I think Ultimate was in development alongside the Switch itself, because they knew that with the Wii U essentially being dead on the water by the time the Switch was formally announced, they knew they had to work on a Smash game ASAP before Smash 4 would become obsolete. I feel that their advantage was that the Smash 4 engine was finished and not only was it polished but allowed for changes, updates, nerfs, buffs and code exchanges, it could easily be re-used for a brand new game.
Smash 4 had to be built from the ground up. They didn't just re-use the Brawl engine and called it a day; they created a brand new engine (which I swear Namco Bandai helped in creating since they know fighting games and they know how to create engines that allow for updates and character modifications). Ultimate I feel just took the Smash 4 engine and modified it in a way that is the absolute best version it can be. It explains why so many content from previous games made it back; the assets for them were already finished and all they needed as an HD remake and balance tweaks.
So I doubt Ultimate could see a massive delay. If they miss anything that's why patches exist. While Nintendo wouldn't allow an unfinished game to be released with the excuse of a day one patch, if the game has issues from day one, they can patch it easily because, again, the engine they are using allows for changes and updates very easily.