I do not know the specific design details of the receiver, but understand that microwave/radar waves are “physically” much longer than light waves. A mirror (reflecting starlight) MUST be perfectly curved and smooth to the sub-micron level. A radar dish - search radar or ground control airport radar - is actually a “weave” of crossing metal bars, sometimes a mesh. You get more detail from a smaller wavelength, but more search capacity from a longer wave. It is very likely the “dish” is actually not really there, but is series of panels of wires or steel bars.
It really is a breathtaking sight-—I came upon it quite
by accident on a hike out of Camp Crozier-—a Peace Corps
training camp in 1963.
I remember reading from long ago that the observatory was constructed in a natural bowl (crater?), with only minor excavation. But as you say, the ground surface is not going to be within 1/10 wavelength or whatever. As far as water accumulation, there must be some kind of natural drainage or the thing would be like Crater Lake.