I'm with mzuzek on this one, if it was coming next year they would've said so, as they'd have gained a ton of hype from that.
There's nothing to indicate it will come out in 2020; it still doesn't even have a title nor have they shown any gameplay.
Thankfully, until during the wait for BOTW, this time we have plenty to play in the meantime.
I'll repeat to you then. It could be that they don't know if it'll be ready in 2020. Let's say the team has a goal of 2020, but the schedule is tight, it's obviously a huge game, and it could easily slip. Nintendo makes no promises that they have to walk back. Nintendo's been very good about keeping a realistic schedule and have delayed only three titles from a promised timeframe in the Switch era. And two of them and it's quite possible that two of them were/will be ready to go, but held back do to scheduling logic.
Basically, I'm saying that the lack of a timeframe isn't really evidence one way or the other of when it will come out.
As for gameplay. Why would we need that? We can just play Breath of the Wild. All they can add are specific new mechanics which can be announced any time. It was probably the reveal in least need of gameplay at all of E3.