A quick read of the rt shadows checkbox note(... terrain or other surfaces...) in atmo and clouds nodes explains this. It's one of the main problems for long render times that loads of new users come across, or used to when the option title looked more important and the warning note wasn't displayed. I think Oshyan must have just been mixed up, I'm sure he knows this.
Clouds will cast shadow in atmospherics, including other clouds, but any solid surface won't unless you tick those boxes in the atmo/cloud node you want the shadows to appear in.