The water need not be potable. Maybe not a great idea for a drought/famine caused dislocation, but maybe for a flood/typhoon destruction of housing event.
The typically adverse effects of using non-potable water on concrete quality would seem to offset the sole advantage of the design over a simple tent - permanence. Flood events in densely populated areas (i.e. those most in need of large amounts of housing quickly) tend to produce particularly polluted water (e.g. Katrina Soup).
At 1:00 in the video they said that they could use seawater and could even work underwater but didn't mention that it had an adverse effect on the quality of the concrete.
Concrete ideally should have clean water for best strength, but it need not be potable. Salts, silt, organic matter, and small bits of trash all weaken the concrete to some, greater or lesser, extent. But the effects can be taken into account in the structural design. For that matter, there are additives that are commonly added to concrete under special conditions, for example to lower the freezing point in winter pouring, that also weaken it in similar ways.
I also know very little about concrete but I would expect that this construction varies slightly from traditional concrete due to the water being sprayed on the outer surface, rather than being part of the mixture. I could be wrong though.
And if the structure is going to be permanent, then using sea water due to a temporary shortage of potable water is going to result in a permanently weak building. I understand that this is supposed to provide a solid shelter during quick relief, but it's going to be much harder/costlier to take down than a tent if it becomes unnecessary at any point in the future.
