Of course mushroom rings are real, you get them when the mycelium grows mushrooms at its edges to spread outwards. But it has nothing do to with EMF.

Eating mushrooms without knowledge will kill you. Cultures either don't eat mushrooms, or they develop knowledge of what mushrooms are safe to eat and which ones kill you - or make you see elves. There's no world where people see elves but don't connect it to the mushrooms they ate 15 minutes ago. It's also not very plausible that they as a culture stopped going on elf trips, but remembered the elves and forgot what made you see them. In short there's just so many ways this is a bad theory.