I wouldn’t blink, either, except that this was a slum (even by Chinese standards of the time), and that it was densely populated. https://en.wikipedia.org/wiki/Kowloon_Walled_City) says it had 5,000,000 people per square mile. Dividing by 20 floors still gives you 250,000 people per square mile.

If I calculate that correctly, that’s ballpark 100 square feet per inhabitant, and that area included shared space. I would guess the average ‘apartment’ would have about half that, and only the better ones would have windows facing outside.

100sqf per inhabitant isn't particularly uncommon in Hong Kong, even in normal flats. It's quite common to fit an entire family in 300-400 sqf and that flat will set you back nearly US$1m on HK Island.