Interesting... I found out if I clicked on the email field on passkeys.io it will pop up with the option to select the enpass passkey. However, if I hit the "Sign in with a passkey" button, it will only try to use the google client as the screenshot in my first post shows.
Browser: Brave 1.60.116. Chromium 119.0.6045.134
Enpass: 6.9.3.892
OS: Android 14