Ultimately one could raise a point that MacOS is also Linux, because it uses Darwin

There's no basis for calling MacOS Linux. There's a legitimate basis for calling it BSD, as Darwin was forked from FreeBSD, but BSD and Linux aren't directly related. Also, Darwin has diverged considerably from FreeBSD, and only a small amount of the stack outside of the kernel shares any code, so it's not necessarily meaningful to think of it as a "FreeBSD distro" in the same sense as you would ChromeOS a Linux distro (which uses, as I mentioned in my previous comment, a more-or-less standard Linux technology stack).

In the current version of ChromeOS, as far as I know, either you sideload Linux or Google completely controls all app stores

ChromeOS lets you install Linux native applications out of the box, although it does so in containers (Crostini, which I believe is based on LXD, another standard Linux technology stack). Once you enable Linux apps, it automatically hooks you up to the Debian repositories, and you can install using apt like you would on any other Debian install.

Whether you consider Crostini to be "sideloading Linux" is a matter of semantics, but fundamentally it's no different from installing containerised LXD/LXC apps on Ubuntu or whatever, which is a common use case for developers and production servers.

For me that is a fundamental conflict with the promise of freedom and user control that Linux gives - with a simple sudo you can be lord of the world.

I think you're making an argument for why it's a bad Linux distro (from a certain perfectly valid point of view), but not that it's not a Linux distro.

There are few if any other distros which are as locked down ChromeOS out of the box, but all Linux distros can be locked down, and if you've ever used a corporate provisioned machine in a workplace or education setting then odds are you won't have any admin freedoms regardless of the distro chosen. Sudoer privileges is something you might have on your own home machine, but not something that you can expect on every Linux machine. Even on devices you own, there are devices that you might buy (such as wifi routers, DVD players, smart TVs) which run standard Linux but which are as locked down (and more) than a Chromebook; it's just that most people don't expect to have unrestricted sudo privileges on their router in the same way as they do a laptop.

For the record, I am not a Chromebook fan. I owned one once for a few years, and thought it was a disappointing, artificially limited experience, and I don't intend to have one again. ChromeOS is not my idea of a good Linux distro. But I'll still argue firmly that it is a Linux distro in all ways that matter.