The fatality rate sounds like a total lie.
In any event I don’t believe it.
The cases were mild because there were known at the time to be at least two strains of the virus, one of which was mild. That’s the one, through the pustules of which they would draw the thread that would later be placed under the skin of the healthy person (often a Continental Army recruit) to have them get mildly sick and recover in two weeks.
Please provide a link to that information.