The beauty of the model is not that YC has a portfolio worth $1B of gain - but that they spent less than $15M to get there: A staggering 67x. Why?
Because they are good at picking Companies for sure - but most importantly because the % they own for the money they put is very high ("founder level" almost)... and is all a consequence of their brand and processes and its power on entrepreneurs, investors + the network effect of their community. Amazing jobs guys!