A kernel is not an OS. Or do you consider Android just another linux distro?
Not really if you look up the accepted definition of OS
Still, Pop!_OS doesn't even have it's own package manager. It's just a Debian derivative like so many other distros out there. I agree that calling it a separate OS is pushing it.
At least distros like NixOS have fundamentally different architectures.
iOS, MacOS, iPadOS, WearOS, etc would like a word with you.