It is very possible. The game seems darker than Botw which was already pretty mature.
The fact that they had so much success with botw they can take some risks on the rating for sure. The game will be a hit no matter what. I mean we dont know if the game overall will be dark but everything about the teaser was giving this vibe. Would be weird that it would be the first thing they show and how they presented it if it's to have something totally different