I've used soil under Black Diamond sand before. I've never had great luck with cories, for some reason, despite trying various different substrates. The tank with Black Diamond was the only time I've ever kept a whole population of cories (pandas, in that case) alive and healthy, with long, healthy barbels, even after two years on this stuff. Take that for whatever it's worth: There are a ton of other variables, so it isn't exactly a scientific study, but yes, it can work, and it's hard to argue with success. I'm all for experimenting and outside the box thinking. The price is hard to beat, especially for a large tank where you might need several dozen pounds of sand. Dedicated fish-safe substrates are notoriously expensive. I've used this stuff in two different setups, both of which ran for several years and had healthy, active fish and vigorously growing plants.
That said, I can't really recommend it, either. It's true that it is sharp and abrasive; it's used as blasting sand industrially which should tell you something. Also, there is the possibility of industrial contaminants in coal slag, since it isn't intended to be fish-safe. Say the machine that crushes the slag has a malfunction and leaks hydraulic fluid or some other petrochemical all over the sand. Who cares, right? It's just blasting sand. But it might kill your fish.
It can work. But you are taking a risk.
