Apple, Google and Microsoft all have native API variants of the Web Authentication API. These typically use entitlements requiring authorization back to a website. This means e.g. a Twitter application could leverage the same authenticator registrations as twitter.com, leveraging both platform and roaming security keys.

>... and should work with my logging into my laptop in the browser and leveraging my phone as a FIDO "key".

The press release details this commitment; for instance, I can use an Android phone to log into a website on my Mac. An example of such an option should be visible on all shipping desktop Chrome browsers if you do a Web Authentication registration or authentication request (I believe unfortunately currently titled something like 'Add an Android phone'). On the Apple side, being able to leverage this is currently sitting behind a developer feature toggle.

One can hope that this will be extended to say the Windows platform itself - at that time, I would expect be able to use my iPhone or Android phone to log into any Windows machine on an AAD-backed domain.