I have met a forensic computer specialist who is deputized by a local small Sherrif's office, he OC's too.
Computer forensics are very open source these days and like you're saying not as complicated as one thinks. To be honest, the OP probably had his threads and user handle marked down by the LE in this incident by simply posting his youtube video... especially if that was in any way, shape, or form used as evidence.
As far as IP address and all that fun jazz, let's just assume for a second LE gets a tip that some person who started a lawsuit is a member here. LE is having trouble proving it though, but they know where the person lives. It's as simple a hosting a picture or website on web hosting you control or even using link tracking sites like
this, and then posting it on here in a cleverly disguised thread that will make most people comment (A.K.A.
social engineering the thread and situation). The LE can then go back to their hosting/link tracker and see what IP's have looked at said pictures/web content and compare those IP addresses locations with known locations of their person of interest via free online tools
like this. Based off of who has replied to their thread, they can start narrowing down their pool of "suspects" once they have a promising IP per the location they're looking for. Sending that pool of "suspects" a PM with more tracking images/links, and you can probably figure out the user handle.
That's the long and sneaky way of digging, other ways include serving warrants to your ISP or OCDO for IP/server logs. Long and short, put something on a network or the internet, it's no longer secure. Now you can be paranoid about this, get your tin foil hat and only access websites using a 256K encrypted VPN connection (your ISP will see only jibberish data at this point) out of a server in some obscure country where US LEA's warrants for server logs mean jack all... or you can watch what you say online and in real life. The second option is much simpler.