Basically he just paid $6M to have a seat at the table and be a guaranteed call when they're raising their series A. I guess that's one way to get in to the early stage game.
He can afford to gamble $6M on YC startups, if they will create demand in the market for his hundreds of millions dollars worth Groupon, Facebook, Zynga shares.
It's a good assymetry position to have.
If one of the 40 becomes the next google, he wins. If he can exit his DST positions, he wins.
However if the web bubble burst, he is sh*t out of luck. And DST will be a huge bagholder.