You can make a case for Garp being injured. But observing the panels, it looks like even Prior to that, they were fighting equally, given both were bruised and removed their coats. You can also make a case that Garp was old, but at the same time, Aokiji has an awakened top tier logia which he could use along side his brawl. The color trio were labeled as the Marine's strongest officers and the WG's strongest military force. So Aokiji being stronger than old Garp, by feats and portrayal, is reasonable.