That's really not true.
The vast, VAST majority of Deck owners will never install another OS on it, and the number of people who will go through the trials and tribulations of turning it into a dedicated Windows PC is practically negligible. It's the same problem that Linux has on desktop, but now the script is flipped.
On top of that, I follow a number of mainstream gaming forums and podcasts, and I'm constantly shocked by the amount of Linux ecosystem talk I've been hearing since the launch of the deck. All the sudden we have non-computer/linux geeks talking about things like Arch, KDE plasma, flatpak, systemd, etc. Sure, they aren't like experts or power users or anything, but most of them were never Windows experts either, nor should they need to be.
Finally, when you look at a lot of the failures of the disaster that is the Asus ROG Ally, I think it's pretty clear that using Windows on a device like this is neither ideal, nor is it actually a big driver of sales. (From what I can tell, the Deck has greatly outsold the Ally, hardware problems aside.)