Of course they're spoilers, don't watch if you don't want spoiled like HOT. Every game released has gameplay demos put out, we've seen plenty of the new Assassin's Creed for instance. The question is do Nintendo spoil too much. For Smash Bros, I say absolutely, their daily content updates were just too much. For Odyssey, knowing how much content are in 3D Mario games, I don't think we've seen too much at all.
(oh and like I said, we've never seen non-trailer gameplay and Treehouse demonstrate things so agonisingly slowly that 30 minutes may as well be ten minutes.)