Re: Or am I missing something?
Oh yes. They were not exposed by TOR in any way.
They were exposed by logging on to a compromised server which then fed them the exposing exploit, which in turn then gathered the information available on the pervs PC. I presume they did not change the server's IP address when it was moved to run the sting.
None of the FBI's information came from TOR except maybe the IPs of the entry and exit nodes (they are not mentioned as part of the gathered evidence) which the PC would of had anyway.