ChromeOS is a Linux kernel with some software bundled on top to make a usable system; it is by definition a Linux distro. Curiously, it's even a GNU/Linux distro, as it uses glibc and GNU coreutils, unlike ex Android which is a non-GNU distro.

I will grant that it's a slightly odd distro, but is it any weirder than NixOS or Fedora Silverblue?

By what metric? It's a gentoo deriviative with glibc, coreutils, etc.