Okay, I get it now.
Oda makes it clear that Xebec's "swordsmanship" actually emulates other instruments like a hammer and a cannon. That's why Mihawk is more skilled than Shanks, because he can produce better cuts (the essence of the sword), while Shanks uses the sword as a completely different instrument, and that's why he will never be called a swordsman.
This is not only not a defeat, but A DECISIVE VICTORY FOR SHANKS.