I find it interesting that you, and @n6h6 below, say more or less exactly the same thing. :-)
This is exactly what I expect in this corner of the FOSS world, though.
Linux is like Christianity: somehow the Unix world encourages schisms and splinter sects who all deny that each other are legitimate. It's sort of a defining characteristic.
Ignoring all the commercial ones, as they are effectively all dead now, and just looking at the FOSS ones, there are at least a dozen or so rival sects: NetBSD, FreeBSD, OpenBSD, DragonflyBSD, Minix, HURD/L9 and its various splinter groups, Plan 9 (9front, HarveyOS, Jeanne, etc.), Inferno, and of course, Linux and its thousand distributions.
Precisely two (2) Linux-based OSes have enjoyed large-scale commercial success as user-facing GUI systems for non-technical users. One has billions of users, the other a large fraction of a billion. Both are from Google, and both share a defining attribute: the FOSS world rejects them.
ChromeOS comes in 2 flavours: there's ordinary ChromeOS, which you can only get by buying hardware built to run it (just like Apple's macOS), and there's ChromeOS Flex.
Flex used to be called Neverware Cloudready:
https://en.wikipedia.org/wiki/Neverware
Neverware grew from Hexxeh's remixed and rebuilt ChromiumOS for ordinary PCs. Hexxeh made ChromeOS Flow, and that is the direct grandparent of ChromeOS Flex: both are ChromeOS for generic PC hardware.
And that, for me, is the significant angle. It shows that this is just another Linux distro.
No, it doesn't say "Linux" in the branding. Neither did Ubuntu for the decade or so it took for it to grab some 75% of the desktop Linux market.
Even today, if you go to https://ubuntu.com/ you must scroll quite a long way down the page before the first mention of the word "Linux" and then it's a negative: that line says Ubuntu Pro (a subscription support service) is more than just Linux.
If someone said "Android is not a Linux" I could see that being defensible. You can't download it for free, you can't run it on your own generic off-the-shelf computer, you can't run ordinary Linux apps on it, and so on. It's a different sort of beast, even though, technically it is a Linux because it has a Linux kernel.
By default and unless cracked, it has nothing else Linux-like about it. No shell, no desktop, no X11 or Wayland, nothing.
But ChromeOS Flex is different. It is a Linux in every way that matters. It's free, it's Free, it's open source, there are remixes and rebuilds. It runs on generic kit. It has a desktop, albeit its own unique one. You can pop a shell. You can run containers on it and in them run any arbitrary Linux app.
It looks like a Linux, it acts like a Linux, and it runs like a Linux. It is based on a generic Linux kernel and userland from Gentoo, and it does Linuxy stuff with Linuxy binaries like any other desktop Linux.
It doesn't have systemd, but thank the great god Torvalds and his apostle St Cox, that is not yet a requirement for a Linux distro. It has `upstart`, which was one of the most widespread init systems before ~Lucifer~ Lennart spread his dark blessing across the land.
It is 100% on-brand for the Linux world that when one specific form of Linux-based OS went mainstream, and is now used by billions, the True Believers of the Linux world disowned it. Android is not Linux. OK, they kind of have a point.
But ChromeOS is a Linux. It's not a typical Linux, because typical Linuxes are nerd tools and that kind of OS will never, ever go mainstream unless someone forces people to use it. (As the government of the People's Republic of China are currently doing, but that is irrelevant right now.)
ChromeOS is a desktop Linux with the Linuxiness stripped out. No choice about partitioning. No weird dual-boot mechanisms. No choice of desktops or package managers: No package manager!
But in every way that matters, it is mainstream, it is commercially successful, it's a good polished end-user desktop OS, and it's a Linux.
So of course the Forces of FOSS hate it. Of course they do.
And how do they express that contempt? By saying it's not a True Linux.