Not talking about all the Linux distros or different versions of Windows obviously. And my definition of “desktop” would be “able to connect to WiFi and launch a modern web browser”, since that would cover 90% of most people’s use cases.
I know of:
Windows
MacOS
GNU/Linux
GNU/Hurd
BSD
HaikuOS
What others are there?
TempleOS?
Your definition is very broad though, a device like a Nintendo Switch would be covered, even though that’s clearly not a desktop 😁.
I’d interpret their requirements as “desktop with WiFi and modern browser.”