I'm not an advocate for either vitasand or calcisand as a substrate but when they say it's digestible they mean in small amounts.
The "rocks" in calcisand you refer to are calcium carbonate, one of the most pure forms of calcium on the planet, so in small amounts it will digest just fine. However the dust from calcium carbonate has been known to cause respiratory infections as well as severe eye problems that can lead to blindness.
I've never personally used Vita-Sand but I have heard of it causing severe impactions if too much is eaten. It is however made from calcium carbonate and other vitamins as well so in very small amounts it is perfectly digestible.
Despite the fact both will digest when eaten in small amounts only, the other health issues related with these products make them not worth using. If anyone is going to use sand as a substrate or even just a sand box for your pets to play in I suggest using washed play sand.