I can't believe I have to explain this but Koby's rank is not an accurate representation of his strength. Koby has clearly been VA level by the time the Rocky Port incident finished. Koby when directly compared with an actual VA the VA got wrecked without doing a damn thing while Koby was scrapping with Boa offscreen. There is also the possibility Sword members can't rank up given that Drake never ranked up despite being a Rear Admiral before the timeskip. You can also straight up turn down advancements like we see Garp do many times and given that Koby is his student..... Oda has offscreened Koby a lot but as of now hes likely in the YC3 range and likely has been every since the Amazon Lily raid.
Also Akainu might just be a petty fucker and is like this boi is piping my daugther so no promotions
Also Akainu might just be a petty fucker and is like this boi is piping my daugther so no promotions