The argument isn’t whether someone is or isn’t a swordsman
Being a swordsman and having other skills isn’t mutually exclusive.
Mihawk is the best at swordsmanship. Shanks is good at swordsmanship, but he clearly has other strengths:
All of Mihawk’s feats are about swordsmanship, he cuts icebergs effortlessly.
Most of Shanks’s feats are haki feats that have little to nothing to do with swordsmanship.