well that's actually classic Hitlerite lore, before the theory of proto-indo-european had quite matured they were calling it 'aryan' after (i think) the first recorded ethnonym from the vedas. they'd already (correctly) gotten to the point where they thought this language was the origin of all european & indo-iranian languages, but hadn't settled on the modern terminology and some details.

so crackpot Arthur de Gobineau enters onto the scene, and married this to racial theories roughly conforming to . all the places speaking indo-european languages that are less than snow white? white aryan paradises that got overwhelmed and outbred by swarthy subjects they'd gifted Aryan language and civilization to. all the still white places were good and the best were the most whitest (germans, scandinavians, in an oddly flattering move for a french racist) but the Danger---what happened to the Aryans in India and Iran, it still remained! the white aryan purity of europe must be protected! you can see why the Nazis liked this shit

actually correct and citationed telling of this here