Its fairly common to refer to Linux distros as an OS, or even include OS in the distro name (Pop!_OS, NixOS, CentOS). Linux is the kernel, and disros add stuff to make a functional operating system.

It's a trademark thing; there are some prerequisites and manual approval involved before you get to officially name your distro "[Distro Name] Linux".

https://www.linuxfoundation.org/legal/the-linux-mark

ChromeOS

SteamOS

PopOS

ElementaryOS

EndeavourOS

ZorinOS

CentOS

SerpentOS

Cosplayers, all of them!

I'd like to interject for a moment...