Because the Wii U, 3DS and Vita are literally from this gen.
Especially the Wii U for home consoles.
You're right. It's not worth arguing over.I think it's just moot to try to categorize things into neat generations anymore. Nintendo is doing their own thing and releasing new hardware off cycle relative to Sony and MS. Cutting the Wii U short and consolidating their console and handheld hardware with Switch further muddled that as console and handheld generations where viewed separately before as many handhelds had longer lifespans before being replaced than consoles.
So to me, Wii U and 3DS are "last gen" for Nintendo regardless of their release dates as they've been replaced by Switch. It just doesn't line up with PS4 and Xbox One that came out earlier and have not launched their next gen hardware yet, unlike Nintendo did over two years ago with Switch.
Like I said, not worth arguing about as generations are much more amorphous now with Nintendo being off cycle and doing their own thing, mid-gen hardware refreshes, the rise of streaming and subscription services, likely more cross gen games than ever before and so on. Hence why I listed the Wii U and 3DS as I'm ok with others including them even though I consider them last gen since the Switch replaced both. It's murky and not worth arguing over especially since some take such distinctions way too seriously.
PS4
PS4 Pro
XB1X
I'm not sure why some are including Vita, 3DS, and Wii U in their lists.