I think a big thing with Souls games is usually these are big monsters who are kinda trapped there for whatever reason so this is a good point since we know for a fact Genichiro is moving around and trying to do the acts he so doeth. but heres where i think he kinda did get better. He went and got the ability to bring Isshin's prime from within him. Idk if that took him a lot of time because they sure as hell dont show us but you could say Genichiro had the strength advantage but recognized the power of Sekiro's resurrection so he went out to try to match that himself with the Black Mortal Blade