Maybe it’s under the skin......................
I’m skeptical. Why didn’t the scientists collect this specimen for further study? Maybe it’s a real cap that broke off a piece of rotting wood and somehow stuck to the frog in that position. By the photographer maybe? It’s a mystery.
I’m just reading Cordyceps doesn’t affect amphibians. Chytrid frog’s. is the only fungus that affects frogs. It’s fatal to them, but they don’t sprout buttons with it.
https://psmag.com/environment/a-deadly-fungal-disease-is-devastating-amphibian-populations
I don’t think it would be possible for bonnet mushroom mycelium to grow beneath the skin and have the fruiting body break through the skin. The frog’s body doesn’t provide the right conditions for mycelium to grow. Imagine that in a human—it’s not possible. It is a very interesting mystery though. My bet is photoshop to gain notoriety.