Technically it's completely edible, insofar that it'll only give you nausea / stomach cramps and a wicked high. Whereas white amanitas are lethal.

The "properly cooked" here refers to well dried and sort of cured material, which has more uh, I want to say "muscimol in relation to ibotenic acid", iirc. Your liver will also convert the ibotenic acid into muscimol, but that's where the nausea would come from, as your liver works hard and there's metabolic byproduct or some such.

But when you properly dry the shrooms, a lot of that ibotenic acid gets tuned into muscimol, which doesn't usually cause nausea that much.