Exactly–ChromeOS being Linux-based is, at this point, an implementation detail. No ChromeOS "apps" depend on that fact, because they all run under their own VMs or ABI sandboxes. This means that Google could make a Fuchsia-based ChromeOS and nothing in userland (save for the code of those VMs and sandboxes) would have to change.

Isn't Fuchsia OS as well, kust no GPL?

Yes, various parts use BSD/MIT/Apache licenses, e.g.: https://github.com/fuchsia-mirror/docs/blob/master/LICENSE

That said, parent comments seem to be talking about two different topics. Later comment about Google switching the underlying foundation of Chrome OS from Linux to Fuchsia sounds plausible, while earlier comment complaining about Fuchsia being proprietary is nonsense.

It might not be proprietary, but if OEMs already only release updates at gun point with GPL, just wait until they get an OS with MIT like licenses given to them. It will be a big party.

Still does not make Fuchsia proprietary. There will be proprietary fuchsia bundles, sure, they are unfortunately standard with Android allready, but that was not the point.

Fuschia doesn't contain any Linux at all, hidden or otherwise. There's a compat layer you can run on top, but it's closer to the Windows subsystem for Linux than Chrome OS.