I understand why (tying to avoid the "yet another Linux distro" stigma), but when you go to "What is Qubes OS?" in https://www.qubes-os.org/intro/ ; they fail to mention that is Linux based.
Well, to be fair this one has a FAQ entry: https://www.qubes-os.org/doc/user-faq/#is-qubes-just-another... ; although they try to understate that they use Linux.
If they want me to trust them, they're doing it wrong.
buy they don't. They want you to look at their hands. This is a security oriented distribution.
For the guests that actually run the software you use directly, they provide templates for multiple Linux distros (and don't have their own brand) and support Windows as well, so I don't think the application-level is seen as part of it. Whereas I'd see "provides a more or less curated selection of applications + configuration" as an important part of a Linux distro.
They distribute Fedora, Debian and Whonix domU guest images (and the dom0 host that is installed is Fedora), but you're free to install BSD, Windows, or whatever else you can get to run as a VM guest.