With Hong Kong's population density, I could see this technology working fairly well. I'm not sure how well it would work inside buildings, but could be good enough for people walking on the streets. It would also theoretically possible for each phone to store hundreds of encrypted text messages if latency isn't important.

This would work exceptionally well if what they're doing is store-and-forward, instead of a classic mesh.

HK definitely has the population density to make it work, especially with people moving around.

This would entirely depend on population density and how often people are in relatively crowded areas. Something like this may or may not work in an apartment building, but it seems like it would definitely work on crowded city streets, particularly if you allow for a transmission delay as it spreads from one device to another.

(And to that end, in a big city like Hong Kong, you probably wouldn't really need massive adoption, just a few % as you are in close proximity to hundreds of people fairly regularly).

I wonder if it's feasible for cities to implement infrastructure to help during times of large gatherings.

During the NBA championship parade this summer every cell network in Toronto was basically shut down, even with extra capacity along the parade route. If there had also been the bandwidth of a series of 'permenent' Bluetooth nodes, I wonder how that would effect things.