For the kinds of devs they've hired, Windows is the third most common development or production environment. I've worked at startups like this for 20 years and haven't touched a windows system.
I realize yall are out there, but from where I'm sitting, this isn't odd at all. They're likely most familiar with and using Unixes.
* The framework they use supports X11 and Wayland out of the box, it wasn't as much effort as you'd think.
* They accept contributions.