Yea I feel like this article is pretty misleading. It would be better if Google officially supported removing the entire ChromeOS and doing a full install of any Linux distro you want. This isn't really running Linux on your Chromebook. It's running a chroot of another distro under their Linux kernel/GUI layer. It's pretty different.

It's not even a chroot. That's how crouton works. Crostini, the google provided solution, actually runs all the "containers" under a KVM virtual machine. So it's even more abstracted.

If this is true, then this is very disappointing.

But, why? What advantages does this offer Google beyond a 'normal' UEFI chain?

It's about achieving physical security against all but highly dedicated and competent attackers. There's a great overview here: https://www.chromium.org/chromium-os/chromiumos-design-docs