Perhaps you should look at the announcement. Of course webauthin is in the browsers. They didn’t announced that they now support webauthn??? I said the webauthn THING because it’s an addition to that functionality!
A 2018 page announcing the base API, and a devtool page about the virtual authenticator debugger is not relevant to this new extensions.
The thing that’s an unshipped proposal is the sign extension.
So my point stands, nothing new and useful in this at all and nothing they announced is usable for anybody today.
Or am I mistaken? What new thing will you, or anybody, do with this 5.8 announcement?