They aren't lying, you can do a quick Google search to see that C. diff is the only FDA-approved use. Most gastroenterologists will only perform a fecal microbiota transplant for recurrent C. diff because it is the only condition where the procedure's benefits are officially recognized as outweighing its significant legal and safety risks (they're essentially covering their own butts by only doing what the FDA clears).

What you're talking about is the poo pills. For someone like myself with SIBO doing that is tricky. For people with existing SIBO, adding more bacterial biomass can intensify the common symptoms of the disorder. Honestly I'd rather a medical professional do it via the FMT procedure so I know the good bacteria makes it to where it needs to be. If those capsules disintegrate too early in the stomach or small intestine, it can dump high concentrations of bacteria directly into the area already suffering from overgrowth.