Actually, those who really believe that Slovenia didn't exist until 1920 or so, just do not understand world history. The Slovenians were in that part of the planet for hundreds of year before that. Actually for over a millennium before that. The Slavs emigrated from around the current Baltic countries starting around 400 or 500 AD and concentrated themselves a couple hundred miles east of Kaernten/Slovenia before moving elsewhere--Slovenia being just one of those elsewheres...
Check this Wikipedia article:
http://en.wikipedia.org/wiki/Carantania. There are lots of other historical accounts of Slovenia, Carantania, and Carinthia. So, this shouldn't come much as a surprise. The northern half of former Slovenia (Carniola--600 AD or so) just happens to be present-day Kaernten...
The Bavarians settled most of what is Austria today by gradually moving out the Slovenians over the past seven hundred years. But the Slovenians never went far away. That why it's easy for them to return.