Earlier quoted context omitted.
Maybe it’s because you have to ship stainless steel from Earth. Maybe the mass of the fungus can be grown from CO2 from respiration. The carbon originally coming from food, which has to be shipped up. Lots of maybes, but you know, maybe...
To grow fungus from CO2, don't you first have to grow a plant then let the fungus eat dead plant? Plants require (heavy) equipment to support them.
If it can photosynthesise (on gamma rays no less), then perhaps not. Mold can grow on concrete walls and stuff, so not sure what it needs.