I don't think generations apply anymore, I think Microsoft is going to ditch them and PS5 will likely be the last for Sony.
But regardless what even puts the Switch into a group with the ps5/xbo2, it's not like they're going to share many games or even be remotely similair hardware wise, they're not even going to release within a couple of years either.
There's no need to throw the Switch into an industry-wide generation for the sake of it. It doesn't fit anywhere and Generations are likely to soon be a thing of the past anyway.