YC has invested $8 million [...] the value of YC’s portfolio is approaching $10 billion, largely due to Dropbox and AirBnB
If I understand this correctly, the $10B is the total valuation of all companies YC has invested in, not actually the valuation of YC's portfolio (which starts at 2-10% and decreases as other investors buy in).
Which isn't to say that YC hasn't been achieving great returns -- but it's not fair to paint it as turning $8M into $10B.