Smash Ultimate is likely to have twice the number of stages of the Wii U version, and UNLIKE the Wii U version, all returning stages have been completely rebuilt with brand new HD assets (aside from the N64 stages, which are probably taken straight from their Wii U incarnations). That means that around 68 stages (pending unannounced ones) have either been completely rebuilt with new assets or are brand new.
In addition to that, 17 characters are sporting brand new assets, including new characters, characters that missed Smash 4, and characters redesigned since Smash 4. And that’s ignoring characters with slight design tweets, like Mario.
Then there’s the new items, new Pokemon, new assist trophies...and whatever the heck those two boss stages with Dracula and Rathalos are supposed to be. As things are, it very much looks and feels like a new game that’s been updated for the Switch era of Nintendo. And not to mention there’s a whole segment of the game’s main menu that we don’t even know about yet.
So yeah...I’d say it most definitely qualifies as a sequel. Just one that’s put a lot of resources into honoring the past. And heck, if this game also ends up having some sort of substantial single-player campaign, which Smash 4 lacked...just icing on top imo.