This is the same Credential Provider API they already have to integrate with to show the password autofill in iOS so there is already _some_ code for this.
1Password _could_ just integrate with the native UI. But they chose not to. This however means shipping a native app which is a lot more heavy-weight than shipping a web extension.
I opened an issue in the webauthn repo about giving an API for WebExtensions to hook into the passkey autocomplete but there hasn't been any traction or appetite for it unfortunately :(