He didn't hold the rank. He never completed school. He was working as a CSM; he was never an E9. He was an E8 in an E9 position. My brother has been doing the same thing (not even promotable) for like 2 years. It's why they "demoted" him on his retirement order.
The Army, Guard/Reserves in paticular, is weird in that we'd do what you're describing with your brother (Frocking) as well as actually promote people before PME and demote them if they didn't complete it.
It's called a STEP promotion now, used to be called a conditional promotion. You get the rank and pay under this, then you'd need to complete your school within 12-36 months (depending on rules) or you'd lose the promotion.
I see this type of promotion much more often than I do frocking.
Doesn't change that he retired as an E8, but it is possible he was an E9 with pay.