They key point is density. The denser the population, the more people need to be immunized for herd immunity to be effective, because the more people the average person comes in close contact with even only in passing.

It’s like the difference in walking six blocks in a sleepy town vs six blocks in downtown Manhattan. Even in “rush hour”, with the sidewalks at maximum typical capacity, the former might net you a dozen close encounters while the latter could easily net you 1,200 close encounters. If you are immunocompromised, the same level of herd immunity in the general population makes the former a much safer environment than the latter.

Statistics can be wild.