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?

Only runs browser instances. Even Crostini is sandoxed in a way similar to WSL2.

User-facing apps are browser instances and VMs, and under that is a pretty normal userland. It's still a distribution of software on a Linux kernel. And for that matter, is Silverblue any less a Linux distro with its read-only root and apps in flatpak/distrobox? Are Qubes OS or Proxmox with everything in VMs?

People buying a Chromebook down at the mall will never see GNU userland.

Folks using Silverblue and Flatpak distributions get to use GNU userland.