That is up to the implementor.
The implementor can ask the browser for certain security features of the authentication device. E.g. is the authentication device the same device as where the authentication flow is happening; is there a biometric check on the device or a pin on the device (i.e. the second factor is there but device-local) who is the manufacturer of the device (with consent of the user; given this is privacy-sensirive info) etc. And the implementor can then make a decision whether a device is 'strong' enough for single factor auth. All this information is cryptographically attested by the device.
You can ignore all that and only use it as a second factor always though. That's totally up to you and how you use the authentication primitives that webauthn provides.