Luffy wasn't using even barrier ryuo and Yamato admitted Luffy was holding back. Clashing with an elephant gun, isn't the same as clashing with red roc, it isn't even the same as dealing with a base elephant gattling attack.
Luffy was rushing to fight Kaido, not have an all out battle and waste stamina with who he assumed was an underling.
Yamato would have died IN CHAINS like Oden, due to her lack of INTERNAL DESTRUCTION RYUO, if not for Luffy. Show some respect to the future pirate king.