The fungus “feeds on” the radiation in the sense that the radiation is its source of energy and life vitality.
Unfortunately this says noting about any ability to reverse radiation sickness in humans. It does not even indicate that the fungus, taken out of that environment, would itself be radioactive — but probably it is.
One Eastern European leader, in the days of the Iron Curtain and the Cold War, proclaimed in a rah-rah speech that “the Millennium is on the horizon!”
In the car afterward, one of his toady bureaucrats asked him in all supposed seriousness if it was true that the Millennium is on the horizon.
The grand pooh-bah replied, “Don’t you know that the horizon is defined as the line that recedes as you approach it?”