What he's really talking about is chord progression (while he doesn't really say that, it's essentially the description of his first few paragraphs). Most university-level music classes discuss common chord progressions. As he notes, the I, IV and V chords are the most common to appear in a song.

It's a great article, but I think he may have done a lot of work to find out something that is fairly common knowledge lol. Still cool though to have the supporting data. (edit: it would be cool to make this an interactive piece of data presentation to help you write songs. Also, the I, IV and V chords are so popular because they naturally make people feel good. It's why they show up so often in 'pop' music. minor chords have a more depressive quality to them)

Also, if you need proof that certain chords show up often in music, just listen to some Nickelback. Here is a fun link (that I THINK works. my speaker only works in the left side ;) ) http://dagobah.net/flash/nickelback.swf