The questioning on whether or not Musashi is strong by defeating Inshun is Musashi clinging onto his goal to be stronger. Hearing that he's basically at the top of samurai that have no master like him is a good baseline for him already, but that is just the bottom feeder section. Musashi wants to be stronger and this first major duel against a prodigy like Inshun is a great way to get him on that path.