That is 100% correct. I just retired June 30th. But i actually did my retirement ceremony April 16th, and then started terminal leave immediately after the ceremony, because i had a ton of leave saved up. Not sure about Army/Air Force, but Navy/USMC we are authorized 10 days of job hunting and 10 days of house hunting that can run concurrent to your terminal leave which what i did as well. So if you have 20 days of terminal, adding in the job/house hunt once approved, you can start your separation/retirement 40 days early. So it's possible that's what's he is doing. You can even sell your leave back and make money off it.
Pending each command's leave policy, some have a limit of how much terminal leave you can take. Most are 60 days. one of my prior ship's i was stationed on was 30 days. My old command in Sasebo Japan was 45 days max of terminal you can use. Not sure what the Marine Corps policy is since i am/was Navy.