This is the first I've heard of the term micro aggression.
That "where are you from?" question I first came across from a comedian. He was mixed race Australian and moved from somewhere like Sydney to Melbourne. When he got asked that question he didn't initially understand what they were asking and his reply was that he was from Sydney. Because he never really knew his dad and the family didn't like to talk about him (yes there was some racism there) he didn't actually know his genetic ancestry on the side that gave him his darker skin. His mother was European though, so his next answer was in regards to that. The poor guy couldn't give them the answer they were after! 😆 I guess its a choice to take offense or not. In this case he had a laugh about it.

Taking offense at things like this means you're making an assumption. What if that woman flinching and holding her handbag tighter on seeing a Latino does so because she was mugged by one? Assuming someone's knowledge could be offensive to that person too. Like you say, not everyone realises that North Koreans would struggle to leave their country, but they know that there is that separation between North and South due to the media. You can't know something you are not taught. So calling their lack of knowledge a micro aggression could be seen as insulting their intelligence which could also be seen as micro aggression.