I have seen this site before. The concepts are interesting, but I would be very careful in taking the website's word for anything.
For instance, just because a dome "roof" has twice the surface area of a "normal" roof, that does not automatically mean that the dome will catch more water. This is a function of the shape factor as viewed from directly overhead...or possibly from the angle at which wind-driven rain hits the structure. As with everything else in life, check the information before you use it. Basically, it is a function of the size (square feet) of the roof as viewed from some angle (usually 90 degrees to horizonal).
I agree that this group is using some interesting construction techniques that are a melding of monolithic dome and earthship philosophies. I also see some influence from the ferrocement crowd as applied by Peter Vetsch and other architects who have successfully used thin shell cement structures without air forms; many of them earth sheltered. I think it would be interesting to see what methods they are using to build their domes before we evaluate the $20K price tag. Still, the number is good motivation to check it out.