It's a view that many Americans have of urban black America, where leaving your house is an invitation to be killed and most young black males are superpredators, ready to rob, rape, and kill. Especially whitey. Cross the border and if you're whitey you're dead instantly. A lot of those people also think that if only Republicans were in charge, and they enacted more tax free empowerment zones and locked up all the "bad guys" (but not the 2 or 3 good guys), then the stain of failed democratic policies would finally be lifted and conservative interracial nirvana would be reached, with everyone knowing their place, of course.
I sorta understand this from people for whom a "big city" is some alien land depicted on scary cop movies and the 6 o'clock news, but Trump lives in New York. Okay, I get that he doesn't really live in New York. He lives in Trump York, a world which contains his absurd condo, his limo, and the various elite places that limo takes him, most of which have Trump in the name somewhere. I doubt he even looks out the window very often. Still, if the New York outside of Trump York really is a post-apocalyptic hellscape, why live there at all?