Substrateless does use substrate. It just means that the grate keeps the eggs from touching the substrate therefore you can make the substrate much more wet. The people that use just water run the risk of have g water slosh on to the eggs which can cause mold spots.
The light diffuser grate I use for my egg boxes was actually sinking in the substrate because I had it so wet. Now I use the stiff plastic cross-stitch sheets, cut to size, underneath the light diffuser grate. This allows me to drench the substrate as much as I want but nothing sinks or sloshes.