Maybe, but I don't really know where they're seeing that type of stuff. The first two trailers were both incredibly cryptic with very little of substance to actually offer, while the third and fourth trailers were just gameplay showcases of Salmon Run and Turf War which, while nice, don't really tell us a whole lot.
In comparison to something like Xenoblade Chronicles 3, it feels like we know a lot less about the game going into the direct, which is kinda silly considering how long ago it was announced.
It's definitely a big title, but I'm not quite sold on the idea that it's so big that it warrants this type of special treatment. I already don't like how long the marketing for Zelda games take, and Breath of the Wild sold twice as many copies as Splatoon 2. Splatoon 3 isn't big enough outside of Japan for me to think it deserves the slow burn marketing cycle yet. If Pokemon and Mario aren't getting these types of marketing cycles, I don't see why Splatoon should get one either.
Also don't agree with your take on Metroid Dread. Nintendo has shown time and time again that they're more than willing to end Directs on "insignificant" games (Xenoblade Chronicles: Definitive Edition, Bayonetta 3, etc.). Ending the February 2021 direct with a game slated for 2021, and then saving Splatoon 3 as an E3 2021 announcement, would've flowed so much better without sacrificing any hype for Splatoon 3.