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.
> iOS and iPadOS share a kernel with macOS, and for all I can tell, so do watchOS and tvOS.
Even worse is that iPadOS and iOS are effectively the same operating system, one ships with a phone dialer and the other ships with UI tweaks for tablets. I would not be shocked if they are built from the same source tree.
unlike, say, illumos, macos doesn't contain anything genetically descended from the v6 unix kernel; the darwin kernel is mach, with chunks of freebsd grafted into it, chunks that carefully reimplemented without copying the functionality in original unix
by contrast, 'chromeos' and android run on fairly normal linux. they even keep up with linus's tree, and though they do have their own patches, they try to get them merged back into linus's tree
