In some sense, it is. Sandstorm called itself an OS too. Arcan calls itself an Overlay-OS.

I think for any platform for general purpose computing that provides an application model that programs are written to target, it's close enough I don't mind. Ie: it makes sense for some end-user software to have a target for Windows, Linux, Sandstorm, etc.

& from the other side- what does Debian add that lets it call itself an OS? A set environment and package manager that software is specifically packaged for.