The concept of a mushroom being generally similar to humans is total horseshit. What they're probably referencing is a mushroom with some signaling protein (or saccride or steroid or something) that is coincidentally similar some human equivalent and your immune system (for some reason) freaks out about it when you eat it. Then, as is referenced, the response to the mushroom happens to also be able to target some of your own cells, and now you've got an autoimmune disorder.

That behavior is not normal for your immune system to do, by the way, otherwise cannibals would all die from allergic reactions to their unfortunate meals. But, the immune system is complicated, so shit happens sometimes.