Again people are missing the point. MC grew up shunned and isolated. Over a decade alone, in childhood. He didn’t learn shit about the outside world.
That being said, his mother surely would have told him stories about demons. If they’re that big of a danger, everyone would know. And backwater people would tell those stories because they don’t have much else to do.
No one is realizing MC is clueless. My life experience taught me early, to realize when someone didn’t understand and to break a concept down in simpler terms. No one is doing that for MC. Everyone around him is the idiot.
Lastly, in a world of magic and monsters, you can’t perceive of a magic mushroom. It doesn’t even take much to realize that the ingestion of the dragon mushroom is the foundation of the poison resistance. The author isn’t great but that’s telegraphed as the catalyst.