You can think of it as a much higher fidelity Stypi, Etherpad, Collabedit, etc, except that you can run the code in the browser as you write it. It really helps alleviate the choking sensation of being asked to write out an entire problem on a whiteboard without any of the modern affordances we've come to know and love.
I loved how smooth and fluid it made the whole process, that they could switch between languages during the interview and the rewind functionality.
Or it could be a simple miscommunication, or they thought somebody else done the app given your performance on the interview. Reaching out and asking will cost you little compared to potential windfall.
Either way, I think you've nothing to lose by sending that mail/picking up the phone. Regardless of the doubts the interviewer may have in you post-interview they'll know that everybody is prone to a bad day every once in a while.