In the article you mention, "five errors" are listed, that Forrest Maready supposedly made in The Moth in the Iron Lung.

This is the first one mentioned in the article:

“Before the developments associated with the 20th century, almost all children were exposed to poliovirus during infancy, largely due to poor sanitation conditions. Sewage entered watersheds without treatment transporting the polio virus into rivers, lakes, streams and thus direct into the water supplies. Indirectly, polio virus passed through the food chain and could be traced even in milk supplies. Due to the low case:infection ratio of infants, and due to protection from transplacentally acquired maternal antibodies, paralysis was rare amongst young children, although the disease itself was endemic. Because of their exposure to polio at an early age, infected infants acquired immunity to the disease thereby protecting them in later life.”

Maready specifically refutes this exact argument in the book.

Many have suggested that improvements in sanitation caused the rise in paralytic polio. It has been proposed that advances in water quality and sewage systems created a reservoir of children that did not gain natural immunity to the poliovirus infection at a young age—as had happened in previous generations. This may seem to make sense on the surface, but does not stand up to even a trivial amount of scrutiny. The illness was called infantile paralysis for an obvious reason—infants were targeted time and again, as were children. It is unclear how improvements to sanitation could simultaneously prevent children from exposure and target them more frequently than anyone else.