I was thinking that number seemed crazy, as it is closer to ~10k at 3km for me(near Cambridge, UK). However, using that site and checking a few friends houses I see Cairo isn't that unusual; ~400k in Camden, 750k in Paris' 11th. And, making a couple of guesses at denser places led me to 1.7 million in Mumbai.

If anything it makes more amazed that regular wifi works as well it does between the house and garden, and I'll try to remember that next time I'm bemoaning its magic.