How? He did not have a large exit to give him a bankroll to invest. Unless you mean from whatever YC shares he received as President, which seems unlikely. Even if he was paid ~25 million in equity per year, did YC shares increase 10x in that time?
Didn't he own like 10% of Reddit. Here what pg had to say about the guy.
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Sam Altman has it. You could parachute him into an island full of cannibals and come back in 5 years and he'd be the king. If you're Sam Altman, you don't have to be profitable to convey to investors that you'll succeed with or without them. (He wasn't, and he did.) Not everyone has Sam's deal-making ability. I myself don't. But if you don't, you can let the numbers speak for you.
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https://paulgraham.com/fundraising.html