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.
I would suggest it could also mean they don't know if they can get it out in 2020 or not, so they don't want to make any promises. I mean, Nintendo would WANT a Zelda game out next Fall to keep attention while the other two systems are launching.
We know that Monolith started staffing up three months ago, which means they're far enough along that other studios are receiving duties. At a guess that means they've moved out of the planning stages and into full production. Assuming they did start in 2017, a three-year development cycle seems pretty reasonable. Can't say it will happen, but I don't see the blurb as confirmation that it'll come out in 2021 or beyond.
Of course, I'm just surprised this got announced before a Mario Odyssey sequel. With the lack of DLC for that game, I was sure they'd taken all the ideas they couldn't implement to a sequel.