Just because the company was sold for a bunch of money doesn't mean the people (yes even the founders) got a huge payout. Liquidation preference and other schemes can make sure that when the investors get their money there's nothing to left to share between other share holders including the founders.

(Not saying that this is the case here, just pointing that out)

EDIT: As some are already pointing out, yes the title says "bootstrapped". I missed that initially, so apologies.