Just because they only show Tom Nook in his HHD company place doesn't mean automatically that
the game is Happy Home Designer 2. They could have shown Mable and Sable in their shop watching
the video on a laptop and it wouldn't have mean that we get a game where we only design and creating
clothes. I'm pretty sure it would be too much to show every single NPC in that clip, so they just picked
one and their choice felt at Nook, since he's (next to Isabelle) a very well known NPC of the Animal
Crossing Universe and also that one NPC you work the most with when it comes to your house and
such. And his line "I've got to make sure everyone has a nice, new place to come home to when
they're ready, yes?" probably means he will continue his job he had in New Leaf, like new game,
new town, new houses, new work to do. It can also means that HHD and it's features are making
a return in the main game, like how Welcome amiibo got some HHD features, the new main game
will get all of them.
Also, let's admit it: Happy Home Designer wasn't that popular and Nintendo knows it. At least the
fans want more a main game then a spin-off and making the first Animal Crossing on the Switch a
spin-off would cause a riot in the fandom and I'm pretty sure Nintendo is not interested in dealing
with angry AC fans the whole time.
Besides all this, they even called the Switch game a "new
mainline Animal Crossing game"
on Twitter back then.
How confusing would it be to call the game a new mainline and then it turns out it's a spin-off.
Nintendo can prepare their butts for a big riot if that would be the case, lol.