This writeup doesn't really get to the point so, the tl;dr He was looking for places to exploit shellshock by googling for cgi scripts. Most of the ones he did find had already been hit by someone using a perl script that made them join an irc channel that was being used as CnC. He also joined it and monitored it. A bunch of different yahoo boxes were in the channel and he saw some of them get rooted.
The servers compromised were more like content servers though, not user data servers? Yahoo says no user data was accessed. But could the affected servers be used to more easily get at user databases from 'inside'?
short answer: yes, definitely.