I really wish they would announce the opposite, that crostini would be coming to Android and phones would launch ChromeOS "desktop mode" when attached to a monitor. ChromeOS feature flags have been seriously fun and innovative the past few years.
The Android Virtualization Framework (AVF) [0] includes crosvm support, so you can run virtually any OS you want. An Android engineer has also done a POC running ChromeOS on Android [1].
Given the big difference between mobile and desktop Chrome, and the fact that this is "Chrome"OS, it can be an interesting story. I don't many people (anyone?) want to run mobile Chrome on a laptop screen.
I suspect it wouldn't be a lot of effort to make android chrome have the full desktop UI. The Chromium codebase is all one piece, and rendering of the UI is already unified between mac/windows/linux, so making the android build render a desktop-like UI is probably a simple job. In fact, I wouldn't be surprised if the right set of build flags can do it already.
The question is will they also throw out the ChromeOS userspace components and replace them with the android userspace?
If so, this is effectively an announcement that ChromeOS is discontinued but those devices will now be migrated to run android.