Considering that no actual files were made available for download, and many of the visitors were likely researchers, there wouldn't be enough to prosecute, but given the implied level of security the actual pedos were using, the FBI could just search their computers and would likely find enough evidence.
>For instance, pedophiles form their own communities and within those communities, a sense of trust is developed.
Or, pedos aren't drawn from the same population that knows how to do stuff securely, they just read a guide or two on how to use Tor. I'd assume you could get "4-7%" of many groups of people to download something and run it, that doesn't show that pedos necessarily are more trusting. (In fact, I'd expect higher numbers from random internet users, thus implying that pedos are less trusting, which I'd expect at a minimum given the fact that they can access Tor).