We don't know exactly how much they're reusing, and they were making BotW DLC up until the end of 2017. It's just the way this was announced... "a sequel is now in development". Last time I heard those words we went 18 months without news and then it was news that they cancelled and rebooted the project.
That isn't to say the same will happen to Zelda, of course, but that wording basically confirms it's not next year. Otherwise, why use it? Everyone will be more hyped if you say "2020", so the only reason they say "in development" is because they won't have it out in 2020. Pretty simple stuff, really. I imagine we'll only get to hear about it again at E3 2020, where there'll be a larger trailer ending with "2021", and then it'll be playable at E3 2021 and releasing in fall of that year.
Believe me, there are people who call me optimistic for thinking it'll come in 2021.
Wrong, BotW 2 was announced with "now in production" which is a later phase in the development process than "now in development".
And I stand by my point. If your game has a chance of coming out next year, you state as much in the trailer. It gets more hype, simple as that. You only don't put a release year on the trailer if there is no chance of it coming next year, because at that point a "2021" is just lame.