He retires by cashing in on the investment he created over the last 20 years. The same thing would have happened if he put the money in a bank, but the bank would have been creating investments with his money istead, and would have returned it to him at a fraction of the gains they made with his money.
But he didn't cash in on anything. According to the email he fires all the people who helped him build the company and then shuts it down. Kind of a cut of the nose to spite the face response if you ask me. The story would have made more sense if he had sold out to a Chinese conglomorate and took the money to the Bahamas.