Well WiiU was this short lived flop between the Wii and Switch. I would include the 3DS, Wii and WiiU owner in the switch to the Nintendo Switch. During the WiiU era many just decided to play on 3DS and never got a WiiU. That core is much higher than 13.96m WiiU numbers. So lets say 10-15% of Switch owner own the WiiU. How many percentage of the current switch owner own a 3ds?
That is of no relevance when talking about if Wii U games are new to Switch owners. The point made was that Wii U ports are new games for a vast majority of Switch owners, that point is true.
I'd add that many of these games are simply more playable on the Switch than they were on Wii U because of the portability factor. And that goes beyond just Wii U ports.
Mario Kart 8: Deluxe is a great example, thanks to portable local multiplayer the game became a major office lunch hour hit. Instead of 4 player split screen locked to your living room, you can have 12 players and everyone has their own screen. Not only that, but if you have less than 12 local players, you can fill up the slots with online players. It's one of the primary drivers for the Switch, with people either picking it up partially or completely because they can play Mario Kart 8 with co-workers during lunch break. For me, it was like an entirely new game because of that alone, and one that saw dozens of times more playtime.
For games like Mario Kart, the added portability was as important (if not more so) than the adding of IR aiming to Resident Evil 4 on Wii.