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". Metroid Prime 4's announcement had to come early because of Samus Returns, the same reason why Fire Emblem: Three Houses was announced so early, because of Shadows of Valentia. Both times it was about reassuring fans that the next big title will be made for Switch, because you know there were people like curl-6 who freaked out over every 3DS game. Whereas Fire Emblem and MP4 were announced with a black screen, BotW 2 got an actual trailer.
Nintendo does not provide any sort of release window for games that are at least one year away, so 2020 isn't ruled out. Given that they announced the game this week, it's certain that they are targeting 2020. Because otherwise they could have teased the next game of the Super Mario Odyssey team to have a big title announced for beyond 2019, and you know that those guys can finish a game in three years.