Honestly, the Aleppo thing does not matter to me. It was a classic "gotcha" minute. It's in the same category, in my mind, as calling ISIS "ISIL" or "DAISH", as the "cool kids", the beltway crowd, have taken to calling ISIS now. It's not much different from the time that the word "gravitas" was the word you couldn't avoid in the media during a prior campaign.
The media has consistently referred to the Syrian refugee crisis (or the related European immigration crisis) when I've heard reports. I'm not that far afield from news media (save PBS where I'm sure Aleppo must be the cool kid word) yet, honestly, the first time I'd heard the word "Aleppo" was during the gotcha moment on Johnson. And I have no problem admitting that.
And I'm willing to bet that less than 5% of the US electorate could've handled the question any better. I'll put the over/under at 5.5% knowing what Aleppo is even after this. Heck, I'll take it one more step! I bet more people answer "brand of dog food" than give the correct answer!
Johnson's and his handler's mistake? Too honest. His response should have been, "What specifically do you want to discuss about Aleppo?" when he didn't have an answer. That's what you do in an interview when you are without an answer -- make the questioner give you more information.