When it comes to being a "World" championship, any promotion in any sort of industry can declare its top belt, trophy or whatever a "World" championship if they want to. In wrestling, boxing or MMA, a championship is a piece of property owned by the promotion/governing body and said organizations can declare their titles to be "World" championships if they want to. It's just that simple really.
Now whether or not any particular promotion/governing body has the legitimacy to make such a declaration is often entirely subjective. For instance, in pro wrestling, hardly anyone could argue that the WWE World Heavyweight Championship isn't a "World" championship in pro wrestling because of WWE's longtime status, success, revenue, exposure, general fame and that they regularly have shows in other countries. Now if it's some small indie promotion that might perform in front of 50 fans per show, then such a claim has no real legitimacy but that doesn't mean that the owner still can't call or view his/her top title as a "World" championship if he/she wants to.
Outside of the Olympic Games, there's no single wrestling, boxing, MMA, basketball, football, baseball, soccer, etc. promotion/governing body that has competition from the entire world. Even in the Olympic Games, an asterisk could be argued as legit because not every sport in the games has athletes from every country competing in them. The teams of the NBA, NFL and MLB don't compete against every other league or team in their respective sports in the world, yet the teams that win the top titles each year are referred to as "World" champions.
So if WWE wants to think of the Divas Championship as a "World" championship, they can and nobody can stop them. Again, considering WWE's status in the wrestling world, there's not much of a legit argument that can be made against it.