Your argument makes no sense.
WSS means the strongest AMONG ALL swordsmen.
It doesn't matter what your swording technique is exactly, if you use divine departure or oni giri, the people who pull these attacks do so with a SWORD and are SWORDSMEN.
Mihawk is the strongest AMONG ALL swordsmen. Every other swordsman, based on this title, is weaker.
Shanks, regardless of his swording techniques, he is a swordsman. Confirmed by Oda.
Shanks is therefore below Mihawk. He is however the closest rival that Mihawk has as mentioned by Oda.
Shanks is Mihawk's rival.
A drawing of all swordsmen by Oda. Shanks is in it.