How can Nazis claim to base their ideology on race and biological determinism, yet most remain completely ignorant about the actual genetic history of the people they claim to represent?

They construct a myth of a “pure Aryan race” yet the Aryans were not a homogeneous group. Modern Europeans are the result of a mixture of Western Hunter-Gatherers, Early European Farmers, and Western Steppe Herders (including the Yamnaya). The phenotypical qualities you associate with mythical Aryans, such as blond hair, blue eyes, and fair skin appeared across different populations and were never exclusive to a single ancient European group. See facial reconstructions of the Yamnaya people.

It makes no sense to base a political ideology on fixed racial characteristics while simultaneously fabricating the actual genetic history of the group you claim to represent, all to enforce a false representation of what it means to be a European. I understand it on an ideological level and in theory I don’t have any major objections to it. But in practice it ends up being too retarded and counterproductive. It excludes people who are genuinely European and have a legitimate blood and soil claim to Europe (a central tenet of Nazi ideology), or "white" countries as a bastardized version, while at the same time can open up the possibility of allowing in people who are not genetically native European. Is purity spiraling over a false ideal self-defeating? I want to be a romantic and a racist, but too many who share those labels are incapable of thinking and easily manipulated into willful ignorance by outside forces.