I' m thinking that the cat would not have played a role in the story at all if he was simply posting messages to forums using TOR.
What many "hackers" do is simple.
Create or borrow a program that will be "useful" to the end user. Something like Perfect Dark or Share for example. Append your malicious code and write a short tutorial including "Don't forget to open TCP port 3953 on your router." Most won't give it a second thought. Because most routers are still using UPnP, you could just write your code to send a request to open the port. The program could also be written to call home to a server. This could be done via tor if it's included in the package (source code is freely available).
People download his software, install it, and it's running on their system. It's new so AV software doesn't pick it up. Software contacts a web page via a regular internet connection that gets the publicly facing IP, and uploads a small txt file so the hacker has a list of IPs he can use.
Once you have gained some control over the machine, even if using it as what is known as an anonymous proxy (to specifically set someone up, rather than it comming from a tor exit node) you can pretty much do whatever you want.
From there it's childs play to set someone up. Yes, it can be done over tor, or even open net, as connections TO the victim machine will mostly appear as noise (such as file sharing) to any ISP.
The important thing to remember when talking about tor, is that anything that can be done over TCP can be done over TOR. Web based email is done completely over TCP connections.