He’s on leave using built up time off/vacation thus he’ll get retirement.
I don’t know about Federal employees, but New York State employees are able to accrue their sick leave prior to retirement, then the time they had on the books once they’ve retired, is pro-rated, and the money value from it, is used to reduce their monthly health insurance costs through the State. Any unused vacation time or personal leave on the books is converted to a dollar sum, and sent to the retiree in a separate check.