That looks awesome. It works everywhere that OAuth does? Can it be used with a password as a 2FA?
Clef is actually 2FA already because it relies on both possession (the device) and knowledge (the 4-digit PIN that protects the app). We're working right now to (optionally) replace the PIN with finger print scanning, when available. Either way, the knowledge (or biometric) portion is much more about asserting ownership of the device (if it gets lost or stolen, you can deactivate online) than as part of the actual authentication process.
It's not 2FA unless information flows between the user and the authenticator through two independent routes. For example, in Twitter's (and others') 2FA, information must flow between Twitter's servers and the user through the Twitter UI as well as through a GSM text message. That's 2FA.
Someone gets some malware on to the phone and gets the run of it. Records the pin, later steals the phone, or is able to replicate the entire device.
This could be guarded against if the pin changed every time and was delivered through an independent channel, which is what 2FA if all about. A complete, undetected compromise of a single device or a single information channel should not be able to defeat 2FA. That doesn't appear to be the case here.