Nintendo showed this announcement on a show that mostly MS and Sony fans watch... They are doing great marketing, telling them: look, this is our console hybrid, it's an alternative to your console, and you can get high quality games like these... They want to tempt owners of other consoles to buy a switch and play the Bayonetta saga, it was a brilliant move. It's not a coincidence that they showed this, instead of any other game like Yoshi, Kirby or Fire Emblem.
OT, it has a chance of selling 1 million. People that got upset because bayo 2 was exclusive to Wiiu, and didn't want to buy one, will for sure be more interested in getting a switch, which may be even seen as a handheld instead of a console.