Swordsmen slashes vary depending on the durability of the material. It's how pretimeskip, Zoro was using certain types of sword slashes against durable materials like steel(shishisonson) which were more efficient than his long range slashes. Long range slashes are weaker than close range techniques from swordsmen. Zoro barely managed to cut through Pica's rock golem with 1080Pound Cannon slash, yet he sent the upper half flying while making direct contact with it with his sanzen sekai enhanced with his COA.
Mihawk only failed to cut diamond with a long range slash not intended to cut diamond. We've no idea what would have happened if he actually made direct contact with Jozu with his special close range technique enhanced with his armament. Hell maybe he could have cut it even with a long range slash for all we know considering his original intention wasn't to cut diamond with that slash from starters, so may possess some quality slashes depending on the durability of the material(just like Zoro)
