The concept of “world war” doesn’t need to mean “most of the world’s population is involved in this war,” not when nuclear weapons are at stake. A nuclear exchange between NATO and Russia is world-shifting in a way that a nuclear exchange between Pakistan and India is not. Calling nuclear war between major Western powers (which will almost certainly have devastating economic and physical effects on the entire world) a “world war” seems perfectly reasonable at that point, even if most of the world is not directly involved.
The concept of “world war” doesn’t need to mean “most of the world’s population is involved in this war,” not when nuclear weapons are at stake. A nuclear exchange between NATO and Russia is world-shifting in a way that a nuclear exchange between Pakistan and India is not. Calling nuclear war between major Western powers (which will almost certainly have devastating economic and physical effects on the entire world) a “world war” seems perfectly reasonable at that point, even if most of the world is not directly involved.