Because Lor D Coast is that strong, the beast didnt leave the encounter unharmed but gained a massive PU after eating Shank's haki infused arm, the fight was legendary and Shanks exchanged several blows with it its just that since we where seing the flashback from Luffy's perspective we couldnt see it because it was that fast. The fish that atacked Luffy as he left his village was a different one, rummors say the Lord D Coast is now guarding the one piece and will be the series final boss, along with Condoriano, Down D. Stairs, sam-san and Enel after training in the moon.