ChromeOS is as much Linux as anything else is. It’s controlled by a greedy megacorp, but so is Red Hat (IBM) or, idk, Oracle Linux. Yes it’s based on an unusual immutable design, but immutable distros are now cropping up out of lots of projects (Fedora, SUSE, Ubuntu, amongst many others, not to mention the Steam Deck). It avoids using the GNU tool chain, but the alternatives that it uses are already used by other Linux distros (like Alpine). It now uses the standard Wayland graphics stack, and is in the process of moving from upstart (a previously widely used Linux init system) to systemd.
It’s hard to come up with a definition of “Linux distro” that excludes ChromeOS without excluding a bunch of unambiguously Linux distros too.