My best guess is that the hole was there before the mushroom grew. It is hard to find a biological species that need so much iron in their metabolism to the point it evolved something that can dissolve pure metal in a matter of 1-2 months. Irons are often used in enzymes and usually the ratio is around a few iron atoms bounded in a structure of thousands or tens of thousands of carbons and hydrogen atoms. To use so much iron implied that mushroom is literally chock full of special enzymes and I doubt it actually is.
More likely, something broke a hole there and the mushroom grew on it and capitalized on the broken edges to scavenge some extra irons but it did not punch that hole by itself.
More likely, something broke a hole there and the mushroom grew on it and capitalized on the broken edges to scavenge some extra irons but it did not punch that hole by itself.