They convicted him on "supporting the transfer of underage pornography", i.e. he ran an exit node that "allowed" the upload of CP to an Austrian image hoster. Apparently, he wasn't protected because he ran the exit as an individual, not a registered company. Most likely, the Austrian authority checked who uploaded the images, and found his IP address, which became the basis for convicting him. He didn't have any of the materials because all those stuffs were encrypted in transit.

He mentioned that law that was used to prosecute him was changed a few weeks later to protect individuals as well. He apparently now ran Tor exits under an offshore company.

In summary, from what he said, he just happened to run an unrestricted exit node that some people used to upload CP.