Just watched it, and was impressed for the 1st half of the video. Then, it struck me that it would not be all THAT difficult to cheat on something like that. I was impressed with how small the "keys" are, and with how accurately he could throw 2 balls to get a chord.
Here's my take:
That "piano" is simply a large switch to play the next tone in a pre-recorded set of tones. All he has to do is get the rhythm right, and the "instrument" worries about getting the correct pitch.
Granted, it's a very impressive display - even if it isn't totally as advertised. The guy's a good juggler, but it seems to me to be impossible what he's doing.
Remember, when it comes to magic, etc. we see what we WANT to see.
Best to everyone.
George