What's the difference? They were involved in what is colloquially known as WWII. They just refer to their specific part differently amongst themselves, but they certainly did have WWII.
I believe the offense is that here, on an English language website with a mostly American and Western European audience, the term used was insufficiently deferential to how Russians would refer to it in Russian.
It's called a world war in english because everyone and everywhere was at war during that time. Pretty much every group has a name for it in their own language.