That’s why I said ‘appears to be’. We’ve literally never heard of Saint Shanks doing anything else of note other than duelling with Mihawk. Every time characters in and out of canon had the chance to hype Saint Shanks’s achievements, they all returned to the duels with Mihawk. Not even an ambiguous allusion to a ‘Rocky Port Incident’ or something like that, it’s all Saint Shanks vs Mihawk and years later, people are still talking about Saint Shanks vs Mihawk.