That's ridiculous, while some of the causes of WWII developed from WWI, to say it is a single conflict is a complete misrepresentation. First of all, the combatants were different. The Kaiser's Germany can hardly be compared to Nazi Germany in terms of its structures, leadership, or goals. Russia had also gone through a complete change in regime. And where was Japan in WWI? Also, many of the causes had changed. WWII grew out of the aggression and ambition of the Axis Powers, while WWI was characterized by nationalistic fervor on all sides.
Also, it's erroneous to refer to the periods of 1815-1914 and 1945-1992 as entirely different from the World War periods, as though they were free from meaningful conflict. Yes, I admit they were periods of relative stability. However, despite the fact that those periods lacked a substantial world war, they still included events such as the wars of Italian and German unification, the Crimean War, the first Sino-Japanese War, (and in the latter case) the Korean War, Vietnam War, Arab-Israeli Wars, Soviet invasion of Afghanistan, Suez crisis..............and so on.
Finally, the interwar period of the 20s and 30s was not free of conflict, this is true. But how did these conflicts relate in any way to the world wars? Sure sure, the Spanish Civil War included Fascism, etc. However, every event is preceded by another event that may provide some cause for what happens afterward. To say that they are the same occurrence is not correct. Yes, I accept the presence of continuity in time, but this is simply not one single war. Maybe it's easy to look at it as a single conflict because of the similarity of countries involved. To lump it all together, though, takes away from the importance and uniqueness of each war in its own right.
I think I've beaten this to death, but let me finish by saying that WWI had a formal ending in the signing of the Treaty of Versailles. In other words, that war ended.