Reading the paper again, I made a mistake in my comment above. The mass attenuation coefficient was at 10keV not 100MeV. Water has a mass attenuation coefficient of about 5cm^2/g at 10keV.
Table 1 [1] in the paper shows the true comparison with water at 100 MeV where it can be seen that there is not a large difference. On a weight basis stainless steel outperforms the fungus.
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...
> Maybe the mass of the fungus can be grown from CO2 from respiration.
That's not a thing. Fixing carbon to a usable form [0] using "general biological energy source" to the carbon pool is extremely difficult (there are only four and a half, counting c3 vs c4 as a half, pathways known) and requires complex machinery to achieve.
[0] assimilating carbon dioxide in general is not difficult, even humans do it but it does not enter the general carbon pool and is effectively only transiently converted out of CO2, and net does not contribute to biomass.
Either you ship steel, or you ship biological material - you're shipping things either way. (And if you can do ISRU, then steel or just plain cast iron is available)
Table 1 [1] in the paper shows the true comparison with water at 100 MeV where it can be seen that there is not a large difference. On a weight basis stainless steel outperforms the fungus.
[1] https://www.biorxiv.org/content/10.1101/2020.07.16.205534v1....