I can think of a couple of good reasons why a person would retire before being eligible for benefits.
He might want to speak out on what he sees as problems in the military. Except for things which are considered secret etc., he is now free to speak out.
He probably figured he could sell his story for enough to make up for whatever he lost while also telling what he wants to get out to the American public.
You can’t talk about classified things just because you are no longer in.