I've never heard that either. Can't be correct.
https://www.bfs.de/EN/topics/ion/environment/foodstuffs/mush...
> As a result of the Chernobyl reactor accident, certain species of mushrooms and wild game are still highly contaminated with caesium-137 in some areas of Germany.
Bananas are also radioactive - those mushrooms maybe an order or two of magnitude more.