This is absolutley true. I lost an 11 egg clutch doing substrateless, they just dried out and died, and I could not make sense of it. How can you possibly get more humidity than having the eggs in a box over 2 inches of water ? When I got my second clutch I tried it again, but the second time around I put a hygrometer in the box. Imagine my shock when the humidity never went over 30%!
Took the lid off the box, wrapped it in four layers of plastic wrap {press and seal and I don't mix} and boom humidity off the chart and healthy babies 61 days later.
Gale