There is no antidote for Amanita poisoning, so treatment is entirely supportive. Drinking that liter of charcoal 30 min earlier would not have made a perceptible difference, given that the author's body was already busy expelling at both ends.

Personally I was more astonished that it took him over five hours after the symptoms started to get himself medical attention, even though he himself realized it was likely Amanita poisoning!