Yes, this is obviously incorrect, the other central powers were blamed as well. And there was certainly enough blame to go around. However, it's not really surprising that this was contentitious (mainly for the central powers, for obvious reasons), because while it is certainly true that Austria-Hungary backed by Germany (or instigated by Germany, depending on interpretations) very much provoked the outbreak of war, it is also true that the Allied powers seemed far from opposed to the idea of fighting a war against the central powers (at least until the full horror of a large scale modern war became apparent).

And while it is technically true that the allies did not solely blame Germany in the peace treaties but included Germany's allies as well, the Allied reply to the German delegation in 1919 certainly seems to put the blame almost eclusively on Germany