1. The definition of populated can be debated. If there are hotels, or jails (need to do revision on census definition), are those really "unpopulated"? That's not the same as census definition of populations. Eg Penny's Bay, or Chek Lap Kok may have no residents either. 
2. Carto doesn't always work. `population=` is not the only factor in what's important. Eg HKI is less populated than Kowloon and NT. 
3. I suspect Carto `population=` only affects `=city` https://github.com/openstreetmap-carto/openstreetmap-carto/pull/1461