Don't exaggerate. Johnson knew about the Syrian refugee crisis and the moment he was reminded that Aleppo was the second-largest city in Syria he talked intelligently about the subject and how Russian assistance would be needed
And he didn't fail to name a single world leader, he failed to recall the name of Vincente Fox (whom he thought of less than 10 seconds after being asked).
People forget names sometimes, and the fact that he was willing to own up to it on camera instead of Trump or Clinton's political tiptoeing around the issue (do you know how many times Trump just starts answers with "I started with a small million" or Clinton tries to pitch "Well as an OUTSIDE candidate...")
It's amazing that politicians have become so good at passing the buck that when someone actually owns up to a slight mistake, everyone regards it as incompetence.