At the time he was allowed to be a double-dipper. He went from the Navy to a job working as a civilian at the Pentagon, which he did for another 20, getting another pension.
It's my understanding you can't do that anymore.
I’ve been retired too long. There are four plans now. The first, for those who enlisted before 1981 is the best. They get progressively less generous. One example of the changes, can get 40% base pay after 20 years.
Go to this site to see the latest. https://www.military.com/benefits/military-pay/computing-retired-military-pay.html