Considering how I got bombarded by that news, I think it was pretty hard to miss, especially if you're getting Nintendos promotional e-mails, too.
Yeah you got bombarded by the news... but you are also active on a gaming forum/s and are much more likely to get the news from whatever sites / Twitter etc compared to the average Joe. Far from every Switch user is active like u and me on the matter. . I am 100% it's a lot more than 3M users. Fortnite has been either #1 or #2 on the popularity search in my region since it was launched back in June and I am guessing that's the case pretty much across the globe.
Heck even Pokemon Quest had 2,5M users in three weeks on the Switch. I just think you are far to conservative on your guess :>