I have never kept discus but I have quite the experience with hexamita and the specific parasites causing it, and all type of ways of treating it...It needs a host such as fish. It can be transmitted via feces and accumulated detritus but it being in soil or being prevalent in soil tanks is just a speculation.....I think not....
....Regardless, you can easily beat it if you know how to, in a variety of ways involving food treatment, temperature raise, or at worst for non eating fish you can treat the water. You can even proactively treat it with the right size of pellet food soaked in solution of 3% water and magnesium sulphate which you can cheaply buy from the pharmacy....Scientifically proven Mg sulfphate digested internally at this concentration kills spironucleus vortens. The food needs to just absorb the solution, not be drowned in it......You feed the fish for 5 days twice a day....So throwing hexamita as an example has nothing to do with discus..It has to do with species of fish, susceptibility, general health, origin of purchase, fish keeping habits, etc.. I do agree though that discus and majority of cichlids are the most susceptible to it. Its nothing to do with soil though....I first encountered this while keeping guppies in a plantless tank with gravel...It was a nasty learning curve for me....When you don't know what the hell is going on...
I guess I should have said it is spread most commonly through contaminated soil, typically by birds. Soil is not required for its presence. Being easily treated really isn't the point, nor was the particular organism. There is a whole host of bacteria, fungi, and protozoa commonly found in soil that are infectious to both the animals we keep, as well as ourselves. Sometimes the treatments available are not safe for everyone exposed. The "fresher" the organics in the soil, the higher the risk. It can be a very rewarding risk, but I personally would risk a tank full of discus, a fish noted for disease sensitivity and for not eating medicated foods, when safer and more beneficial choices are available, such as mineralized soil. There are always exceptions of course, but when answering questions online, I tend go with the safer recommendation.
All my opinion.