DDT contribution: not truly debunked. Doctors at the time and since have pooh-poohed it as "denying germ theory", but it has since proven out that it was a reasonable question to ask and there is evidence that the DDT contributed to cases of paralyzation, if not cases of polio. In other words, there is no doubt that Poliovirus causes Poliomyelitis, but not all cases of paralyzation were necessarily caused by poliomyelitis. The data is incomplete, however, and I'd be willing to concede that it's hard to know what if any effect the DDT had on the situation.

However, I cannot link to the first source and considering the second has a high likelihood of outright dismissing evidence it doesn't like, this doesn't really break the DDT case.

The problem with discussions about polio is that it's usually assumed that if you try to see other causes besides just "the germ caused the disease", there is a correlating denial that the polio vaccine had an impact on halting the epidemic. Hence why the other factors are all erased from the explanation normally except that a germ caused the disease, the disease caused the paralyzation, the vaccine stopped the disease, end of story.