FIDO authentication, of which webauthn is the successor, works like this: your secret is a signing key for a digital signature cryptosystem. When you authenticate, it signs a message containing various things including the hostname of the site being authenticated to, and because this is under the control of the browser, a phishing site can't fake it easily (also the browser will throw a fit if you're not on https).

The result is that if you are tricked into entering your credentials into google.com.totallynotaphishingsiteipromise.evilsite.com which is doing a man-in-the-middle attack against the real site - maybe with a let's encrypt cert so they can do https on their own domain name - then the authentication token they send to the real google will have the correct signature, but the wrong domain name.