Not everyone watches an event like e3. Considering the two million number is only for the first day, I'm sure many people didn't know that Fortnite was on the Switch that day
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 :>