I think he's not just talking about the dollar value of exits, but the value of the founders.
The founders of Loopt, the Reddits, and Justin.tv have done a lot for the YC program (and for startups in general) going forward; arguably these 3 moderate successes financially for YC but having very involved alumni might be worth more than a huge financial success which didn't stay involved with YC.
The comment that I was specifically replying to was this:
"Wow, that's phenomenal. In 7 years, out of 8 companies there were 5 acquisitions (and a couple mergers). I'm sure any VC would be ecstatic to have that kind of success rate."
I don't see how that statement has anything to do with the value of the founders.
He said "5 acquisitions" not "2 (or 3) acquisitions and additionally they have very involved alumni.
The founders of Loopt, the Reddits, and Justin.tv have done a lot for the YC program (and for startups in general) going forward; arguably these 3 moderate successes financially for YC but having very involved alumni might be worth more than a huge financial success which didn't stay involved with YC.