Just some bloggers who have always been reliable when it comes to Zelda leaks/upcoming info, etc.
I think considering they're using the same exact engine and are showing us a teaser during this year's E3, I don't think it's beyond impossible that we can see the sequel next winter.
Also, and I'm definitely no expert,. But usually when developing a game, the cutscenes/non-gameplay visuals are usually created and fine tuned towards the ending of game development, which to me would make me believe that they're pretty far into the game itself, if we're getting this kind a full on teaser trailer.
Please be as skeptical as you'd like. I never said I have definitive proof, I'm simply stating there's a high possibility we get the sequel at the end of next year