TBF at that point in history most of the differences between Germans and Austrians were that they had been ruled by different dynasties inbred bastards. They spoke the same language as the Germans and many Austrians understood themselves to be part of the German people. Modern Austria was made from whatever was left of the Austro-Hungarian empire after all the non-germans had left and had no history of being a territorial entity.

In order not to make Germany benefit from losing the war they had to explicitly forbid what was then called "German Austria" from joining the German state in the Versailles treaty since unification with Germany was a popular wish at the time.

This pan-Germanic sentiment was also part of the reason why the Anschluss, the Nazi annexation of Austria, would meet so little resistance and so much popular support a few decades later.

Hitler was German. Austria-Hungary was a multinational empire under German and Hungarian rulers at the time Hitler was born but there was never a separate Austrian ethnicity or anything like that. After WW1 Austria was simply another German state and that's what drove the idea of the anschluss.

I didn't refresh my page so I didn't notice SoyViking wrote a longer and better version of my comment haha