For this mushroom, it's certainly worse than that.
It seems likely to me somebody quoted him the "without medical attention" survival rate.
> "Mushroom poisoning is more common in Europe than in America.[71] Up to the mid-20th century, the mortality rate was around 60–70%, but this has been greatly reduced with advances in medical care. A review of death cap poisoning throughout Europe from 1971 to 1980 found the overall mortality rate to be 22.4% (51.3% in children under ten and 16.5% in those older than ten).[72] This has fallen further in more recent surveys to around 10–15%.[73]"
It's not like the doctors, nurses, and other staff are at liberty to release medical information about the other patients he encounters. So all he heard, at best, were anecdotes.
The death rate for Mushroom Poisoning in the US was apparently something like 2.9 per year since year 2000.
He's really just discounting the 99.something% who survive for effect because two died in the hospital he was at. If that is even true.