2 years late, so I write this for future readers:
Even if that is true, Shiwoo's martial arts was crippled by his master. In the Murim, where strength determines your station, that is a fate worst than death.
Nine Arts Dragon sabotaged Shiwoo's efforts to get strong, which was to protect the people he cares about and so he wouldn't bow down to anyone. You can say Shiwoo doesn't need martial arts to have the courage to stand, but the reason we can keep standing is because we have hope that things can get better, and that is what his master directly attacked.
Nobody but Shiho could understand what could've motivated Cheunwoo to take that choice of action.