Well it's just a matter of history. The term world war was only coined a few years before WWI and as such even given the term can be applied to many, (Napoleonic wars come to mind) they aren't named as such as they by that time had names fixed in the collective parlance.
Error: World War One was called the Great War at the time, and only became World War One after WW2 got going. Point is, these designations are given by politicians and historians. It doesn't matter when in the past someone "coined" a term. The only thing that matters is how we choose to use it.
As for the notion that ANY dictionary possesses the AUTHORITY to declare what should or should not be considered to be a world war, is ludicrous. Dictionaries don't even have the authority to declare what the "real" meaning of a word is. They are REFLECTIONS of our society, not authorities over it. If a dictionary failed to properly define a term like "world war," that doesn't mean that what is or isn't well described as being a world war has changed. It just means the dictionary writers blew it at performing their primary job.