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.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/