radiation adsorption occurs at an atomic level too fast for anything chemical to happen WRT the adsorption, so a peculiar arrangement of atoms can't have any effect on overall shielding levels other than bulk density (like how much "stuff per inch" depends on what stuff is). The hack of the system is the very peculiar arrangement of atoms for this specific fungi can make productive use of the gamma energy which most living things can't do. From a nuclear standpoint long after the gamma is ancient history the chemistry can get started (theres a big timescale mismatch). A plain old mushroom or any ole biomass will block the gammas as effectively as this one. Of course a plain old mushroom or biomass can't live in that location; nothing to eat.

It would be like building a firewall that acts like any other firewall, except under DDOS conditions it uses a whole network of diodes and stuff to store up the electrical energy of each attack packet. This doesn't work with routers because they take maybe "hundreds of watts" to run but the received laser power might only be "hundredths of watts". For a fungi that grows slow enough and has no competition in its niche, gammas are apparently enough.