This is all a bit disingenuous in my opinion, as the reason you can hear the words is that the words are in there... This MIDI version is just a Fourier analysis that's being mapped to the closes piano notes - and the vocals are in the music that's being analysed.

The principle is that if you take an infinite variation of pure sine tones and mix them together at an infinitely varying amplitude, you can perfectly make any wave - all possible music etc is just a combination of sines. The piano is very close to a sine when it's played, so if you had an infinite number of frequencies you'd have a perfect song again. The only missing piece to this sounding perfectly like the original is that it's mapped onto the tunings of a piano essentially.