I recall that the interviewer asked that without context, after they had been talking about prescription drugs or something. (And it sounds like a drug name: "Do not take Aleppo if you have heart disease or liver problems....") It was a weird gotcha question. The guy asking the question knew that a Libertarian would do nothing about Aleppo, and he was asking that question to make Johnson look bad...as if he doesn't care about people in other countries. So, when Johnson fucked up, they really made him look bad.
A more skilled politician would have asked a better question calculated to get information on the topic without exposing ignorance, but Gary was too open and honest, thinking he would be liked for that, which was very foolish.
And, if that had been Hillary saying "What is Aleppo," the interviewer wouldn't have said, "Your kidding." He would have simply said, "I'm asking about Syria, and especially about the violence that has been recently reported in the Syrian city of Aleppo." And she would have given her standard Middle Eastern answer, and we would never have seen the interview, much less be talking about it now.
But, since Johnson was a Libertarian, the reporter left him hanging and ridiculed him. Par for the course with a shit media.