To continue from Comment 5, the URL linked in this bug  provides an API for performing FIDO U2F, but has no mechanism to accept a web browser origin. Instead, origin is always set to something of the form
android:apk-key-hash:<string>. This is similar to the not-privileged FIDO2 API (which we use for testing). Without being able to set the origin to an actual web origin, it's not suitable for websites generally. A website has to explicitly whitelist a given application, which wasn't meant for the general case, rather for in-house apps: https://developers.google.com/identity/fido/android/native-apps#interoperability_with_your_website
Going back-and-forth with Google, they pointed out that WebAuthn has the AppIdExtension for backward compatibility with U2F
sign, but there is no compatibility for the first step,
register, which would make for a difficult-to-use implementation.