You really don't get it, huh?
Zelda stands so tall that in the real world people won't even consider which other games and how many the system will launch with. The decision to buy Switch will have already been made before that point. This is what a killer app is.
Read this: https://en.wikipedia.org/wiki/Soft_launch
Switch does not match the definition at all. Switch also does not have a weak launch, because it's getting out of the gates with a killer app that is followed up by a massive multiplayer hit in the following month. It doesn't matter that Zelda is also on the Wii U, because the two versions launch day and date, the Switch version is superior and nobody is going to buy a Wii U at this point. It doesn't matter that Mario Kart 8 was on the Wii U, because the Switch game is a complete package of the biggest Mario Kart game to date and all the sales data we have access to shows that remasters of blockbuster hits are comfortably among the topsellers for a new system. That's two killer apps in the first two months that hit different spots: One is a huge singleplayer game, the other one is a multiplayer champion.
Yeah, Zelda is such a killer app that the GameCube had two of them yet was still outsold 7:1 by the PS2.