Hackworth is what we call in the Marine Corps a " Mustang" meaning he was former enlisted so for him to go from E-1 to 0-6 is not something to be sneezed at. ( I think some of his opinions of late seem a little derailed..)
Being selected for the senior ranks is highly political. One has to have had the right tours, schools,command etc to be competitive for the Flag Ranks.
Okay, whatever, you know what his title is better than I. But I was just re-iterating what he had to say!!