The only hole now is that the steam ID is probably unencrypted, so malicious users can probably troll others by posting the error message on Steam tech support with their victim's steam ID. But since Valve has the balls to release this info, they probably already have some mechanism in place to prevent these trollers from doing so.
I assume they would simply compare the SteamID posted in the forum against the your Steam forum user name, and would have some questions for you if you were posting someone else's Steam ID. Also, while I'm not 100% sure, I think they might have a way of checking what you currently have installed in your Steam folders on your system when you connect, so they could also check if you have Garry's Mod installed, but are not on the list of purchasers.
The cops can come knocking on my door for no reason whatsoever; that doesn't mean I'm going to jail.
True, but it would still be simpler to avoid the hassle and accusations altogether. Most college students generally don't have the spare cash for a defense attorney if they end up facing an overzealous prosecutor who decides to press charges. And if law enforcements decides to confiscate your computer equipment as evidence to prove or disprove it was you (since their warrant after tracking your IP would probably call for taking your computers), you're SOL if your term papers and other assignments were all stored on it. Even if you weren't responsible for what caused the original subpoena for the IP, a lot of college students (and other people) probably have at least a few things on their computer that would have questionable legality.
Our business in life is not to succeed but to continue to fail in high spirits. -- Robert Louis Stevenson