Don't they already have things like this where the program fills in the background music?

The Baby llama in red pajamas YouTube post had a comment from someone who suggested they used a program like that to fill in the background (and it was pretty good)

I don't think that's what this is.

This is specifically adding dynamics to a pre-programmed track of notes to make it sound as if a human was playing them (meaning not every note is hit with the exact same intensity; certain notes are emphasized to reinforce the melody/musical phrasing).

In other words the computer didn't generate the music; it is trying to simulate a person playing an already existing piece of music. What makes it interesting is that the computer must "understand" the way humans perceive music (to some basic level at least) in order to do this.

This:

Ludacris Llama Llama Red Pajama Freestyle