S-1s only list >5% stockholders - you'll note that the co-founders have been diluted down to 7.1%. Y Combinator has definitely been diluted below that amount, that being said random back of the napkin (aka pulled from my ass) numbers for their share count is probably 700,000ish so their return is pretty damn good.

You won't see them in there because their percentage is too small to be listed (under 5%).