This actually raises more questions than it answers. How does Ubisoft get this number? Because, you know, Mario+Rabbids isn't an online game. it can be played on Switches that never go online at all.
And differs this number so much from actual sales? Because, how many people per household/switch do play it? It isn't your usual party game or something that is shared between the family. These tactics games have usually a smaller fanbase, which means often only one person per household. Overall a strange thing to announce.
I'm sure the number of Switch consoles that never connect to internet aren't statistically relevant in this day and age.
Even if don't play online, Nintendo surely have a background process to map your offline activity, it's an standard process in Big Data world. This data is then send to Nintendo servers once you connect to internet
But you are right that might be some number of profiles that never went online so Nintendo couldn't send reports of their gaming activity