Leave aside all this "Dai Kengo, Skill, aCoC is not infused in swordsmanship,hakiman, etc" shit all you have to do is to ask a japanese guy about the meaning of the translation of the last chapter.
He will answear you that Oda clearly stated that Mihawk is superior to Shanks, he does not indirectly imply that Shanks might have some other secret power making him more powerful.