Giving yourself a title that you haven’t earned has nothing to do with grammar - it has everything to do with honesty, which is a completely separate matter. And any media critter who was anything other than a Democrat butt-kisser would have (and should have) called him out on that.
Plus, a Command Sergeant Major/Master Sergeant with 24 years of military service doesn’t know how to talk about military service in retirement and in written public speeches?