did they implement the linux system call interface or come up with their own? how linux-compatible is it? it must be pretty complete if it can run firefox
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 is true that macOS is a bad example here because it is not using a Linux kernel.
But Apple also names operating systems that use the same kernel different "OSes": iOS and iPadOS share a kernel with macOS, and for all I can tell, so do watchOS and tvOS.
An operating system consists of both a kernel and a userland. The Linux kernel by itself does not make an OS. I think this makes it legitimate to call CentOS, NixOS, SerpentOS, iOS, watchOS, etc., OSes.