Shanks learned Kamurasi when he was 15 yo, so what's wrong about Shanks being close to his prime? This was 2 armed Shanks who already had several duels with Mihawk. 2 years later he lost his arm and Mihawk no longer accepted duels with Shanks.
Wait... Isn't this huge Mihawk downgrade as well? Because if 2 armed Shanks > has been Shanks, it'd mean Mihawk wouldn't be able to bypass Harald's barrier either. But if current Shanks >>> flashback Shanks, it means he's got several times stronger despite 1 arm.
Damn, Harald and Loki singlehandely destroyed so many agendas. It's crazy how 2 giants from a single arc caused so much powerscaling chaos.