Which includes a lot of people who owned a WiiU...
Nope. You would have to assume that all Wii U owners are early adopters of Switch, but Wii U sales were spread over four and a half years, so that doesn't make sense. It would be more realistic to assume that the number of early Wii U adopters is equal to the number of early Switch adopters who owned a Wii U. So right now you are looking at 3-4m Switch owners who owned a Wii U while 10-11m Switch owners didn't own a Wii U. The latter group is more important because it's much bigger.