Its not complicated. Shanks and Roger are same. Shanks isn't WGS same way Roger isn't. Being a swordsman or not is irrelevant.
He can be a swordsman and still stronger than WGS via Haki level difference. WGS is based on sword skill, that is why Shanks's sword skill is compared to Mihawk's because Mihawk's sword skill is the best. Shanks and Roger's ability is based on Haki level, they can do the same thing with any other weapon, axe, club etc But swordsman like Mihawk need special sword to amplify their cutting techniques. Imagine Big mom saying ''look out for Shanks's sword'' not Shanks