Glad to know your doing it successfully. I just want people too make informed decisions. It sounds to me like it is not the best idea, not the worst. I'm not sure why the author wanted to use sand and not cement. Maybe the author was looking for a less expensive floor system than cement.
Cement is alot better.
I would think it is the rock which makes the cement so desirable. Embed the hydronic tubes in this. In the areas where the hydronic tubes are embedded, use bricks on top as the floor. The rest of the floor could either be cement or brick.