He’s still active duty, so his appointment still counts as an order. Basically meaning regardless of whether or not he actually wanted the position, it wasn’t his call to make. No wonder he agreed to it after Harward turned it down.
Mr. B just explained that unless he submits his retirement paperwork, what Trump says goes. Being active duty means he’s bound by UCMJ and he has to follow orders regardless. It’s good that he’s a strong headed independent thinker, but also this doesn’t bode well.