My immediate suspicion when reading the title was that this would be a temporary effect, with the fungi providing a buffer by storing the mercury but eventually releasing the more toxic organic forms. But it turns out that they do transform the organic mercury form (at least methyl mercury) into the less toxic and volatile elemental mercury!
I had not learned about the mercury demethylation pathways before, and as far as I can tell (2021 review) the research on it is still relatively new, with many unknowns. Very cool stuff, thanks for sharing.