They probably had some nice checks written with their name in the "pay to the order of"... but they weren't the ones doing the selling, negotiating the price, or having any say in it more than any other shareholder (weighted by the shares and voting status) would.
Joel left in 2019 https://twitter.com/spolsky/status/1111267189133316097
Jeff left in 2012 https://blog.codinghorror.com/farewell-stack-exchange/
I'm not sure that it is fair to say that the original founders sold it for that amount.