No, you don't get it. When you trademark something, you trademark it to specific items. Since WWE is a wrestling company, superstar was/is trademarked to only be used for WWE related products when it comes to wrestling and only when it comes to wrestling. Any pro-wrestling based product (be it PPV's, TV shows, video games, board games, DVDs, CDs, action figures, other toys, or anything else) no other WRESTLING company or non-wrestling company making a wrestling related product can use the term superstar. Anyone else can use it, as long as it is not another wrestling company or used in a wrestling based product. The movie Superstar was made by a movie company with no affiliation to wrestling and the movie itself isn't about wrestling, so there isn't a violation of the trademark.
You're obviously thinking why a word can be trademarked even though the word was around before the company. It's just like in "the real you" thread....Showstopper was wearing a shirt that said Affliction, had Affliction designs on it, and cost less than a regular Affliction shirt, but it wasn't officially an Affliction shirt. So in essence Showstopper bought a shirt that was a worse quality than the original and that company made money off of Affliction's name. They stole Affliction's designs and "drawing power" to get money that was intended to go towards Affliction itself, thus they stole they money from Affliction. Now, International copyright laws are complicated which is why they can do that, but if it was in the U.S. that company would face a hefty fine and probably even be forced to shut down. The whole point of trademarks are to protect other people's products from being ripped off and thus ripping off consumers.
In WWE's case, they don't want wrestling fans to hear the word Superstar, think of WWE, and watch a different product than WWE puts out, but relate it to WWE because of that word, so their opinion of THAT product would then be their opinion of WWE. Also, superstar was trademarked by WWE before Jeff Jarrett and Vince Russo even left WWF to destroy WCW, let alone before TNA's conception.