From a legal perspective, clearly it isn't the smartest move to tweet your revenge assault on a mugger.
That being said, the muggers are in a weird scenario where they would have to concede that they mugged Renzo in the first place in order to put themselves in a situation to litigate against Renzo. Mathematically, it would be financially worth it but you would have to expect them to understand Renzo's worth while they would also have to be lucky enough to be aware of MMA media outlets in order to learn that Renzo Gracie beat them up and tweeted it. Oddly enough, if they had bookmarks to any MMA outlet that reported the story, they would probably have known who Renzo was and wouldn't have jumped him in the first place.
It's sort of like that football player that Roger Huerta beat up. In order for that guy to sue Roger, he would have had to admit to beating a woman. That's the advantage of beating up people when they commit a crime, they usually won't rat you out and admit to their own crime.