We can speculate until the cows come home about what might have been but the fact remains that while he plays a Gunny on TV in actuality he's a Staff Sergeant. By the way, he would have had to make a lateral move out of the wing to make it to Sergeant Major.
BTW, an honorary rank, degree etc. does entitle the person to use the term.
It would be less than honorable if he pretended it was earned but he doesn't.