Get to know him and you'll immediately realize why.
Unfortunately that's not an option for most people reading this.
Fortunately his qualifications are largely irrelevant for most people reading this. ;)
Maybe trust YC with who they make partners? They seem to be doing just fine so far...
The two comments below (and the others that will follow) aside, I was thinking the same thing. The article doesn't really paint his qualifications well.
Nor is it really important that people reading this understand.