Actually notice how Garling didn’t call it a promotion, he called it “new orders”. In fact it is implied he still has his supreme commander position with the HKs
Imu told him to take a failed Gorosei’s position is all. And he is doing it while mocking the other Gorosei underneath him (bc Imu’s order >>>>> them). Gorosei dont control HKs they are a separate faction that mainly deals with politics
I ain’t do that. Oda did it himself. Gunko can access Imu’s power personally as his vessel she stomps Gorosei