"It's a first, a ground-breaking achievement!" Er, no, it isn't.
What about "the Xamarin-like thing of allowing developers to fling a single codebase at multiple platforms"? We've been doing that for years. It's called Java, or it's called Mono. (What Xamarin did was to port Mono to Android.)
And yes, Microsoft is now touting their "multi-platform Dot Net Core!" While Microsoft claims, as is their wont, that they got there first, we've also been doing *that* for years: one could use Mono to run ASP.Net apps on Linux five or ten years ago.
I *like* the new Google stuff, but the idea isn't new, nor is the capability new. Let's see whether it represents an improvement, and let's see whether they support it.