I think that’s a Thiel thing. Tolkein was was one of his obsessions growing up. It doesn’t surprise me that he saw a plot device intended to be a warning as a really cool idea…

That’s 17g or 17cc per cubic metre, not 17 litres!

In dry countries the relative humidity can be low - sub 40% is common. That said, the absolute water carrying capacity increases rapidly with temperature, so at 40C, the saturation mass is more like 50g/m3 and at 40%RH it still carries more water than 100%RH air at 20C.