So you're pretty limited in choice for d3o tailbone protection. In my opinion the RED total impact shorts provide the best protection, but are naturally the bulkiest out of the d3o shorts. The sessions bottoms also have a tiny bit of tailbone protection, but it's really small coverage.
I think what you'll find is that the d3o stuff is lower profile, and less "cushioned" than traditional foam padding. The dainese shorts you're looking at have a hard shell with soft foam underneath it, which will make it a bit thicker, but will be more comfortable when you take a shot in the ass. When you take a hard impact with d3o, the impact feels "heavy," even though it's been fully mitigated, because they're not usually backed with really soft foam that makes you feel all warm and fuzzy.