Linux

Sorry beforehand for the wall of text.

Even if FreeBSD and MacOS shared the exact same kernel, I don't think that people would say that MacOS is a *BSD - due to what lemmyvore said, it does not work like a *BSD. The code itself doesn't matter that much, and this is not just about ideology (more on that later); it's about how the software interacts with the user, the machine, and the rest of the world.

I'll highlight this through a thought experiment. Imagine an OS called DuckOS.

Let's say that you refactored its source code, compiled the result, and released the refactored version as NaN-OS. No DuckOS line of code was left untouched; you refactored everything. But both DuckOS and NaN-OS still behave exactly the same, they run in the same machines, you even reimplemented DuckOS's bugs and quirks.

For all intents and purposes, NaN-OS is still DuckOS. It quacks like DuckOS, walks like DuckOS, then it's still DuckOS.

Now let's look at ChromeOS vs. what people usually call Linux (the 3%):

• ChromeOS: thin client OS with mostly web-based interface, running software from someone else's computer ("the cloud"). Your data is also in someone else's computer. You can access both from other ChromeOS devices. You aren't really expected to mess with the system itself. Mostly intended for secondary devices, that become fancy bricks without internet.
• Linux (the 3%): new software is installed in the system itself, as usual. Your data is by default stored locally; you could store it in someone else's computer like the above, but that would be an atypical. Intended for generic purpose machines, from supercomputers to toasters. Customisability and adaptability are some of its appeals.

Sure, ChromeOS runs the Linux kernel... and then what? It doesn't behave like that collective mess of Linux distros.

Now, regarding ideology. In another comment, you mentioned that the author criticises the FOSS community for not calling ChromeOS "Linux" for ideological reasons. I do agree that ideology plays a role here, but it's neither a factor that should be brushed off, nor the sole factor. When the FOSS community says "use Leenooks!", they aren't saying "use the kernel" - you can bet that most would be happy campers if you used hurd instead. They're instead saying "use that bag of customisable and adaptable OSes, built upon open source in letter and in spirit". It's hard to claim that ChromeOS is part of that bag, even if it happens to use the Linux kernel.

(And for the sake of statistics it's simply easier to list them separately, as Statcounter and others do.)

Ah, on immutable distros: depending on how things roll with them, might as well argue that they're a third, different thing. I'm not knowledgeable enough on them to say anything.