you can't say they managed keep their race pure . its unhistorical and unscientific statement similarly to Nazi racism.

You can. Because they did. All you have to do is look at their DNA

Obviously no races are pure. But you can claim purity from a certain reference point, the same way you can say that a recipe needs "20% mayonnaise" even though mayonnaise is inherently a mixture of ingredients.

The following is a list of Indoeuropean Steppe-ancestry fractions for the Brahmins vs. the non-Brahmins of the given state:

Tamils: 20% vs. 3%
Bengalis: 25% vs. 12%
Gujaratis: 26% vs. 14%
Uttar Pradesh: 27% vs. 15%

Tamil Brahmins are basically 75% identical to Brahmins from Uttar Pradesh, meaning only 25% of their ancestry comes from actual South Indians.
In fact, Brahmins of any Indian state are more related to each other than they are to the actual people of the state they reside in.

Now ask yourself: How does such a population stay that pure and distinct for 2000 years? Or 4000 years, in the case of the northern states? The only way to do that is through extreme casteism.

Why did the South have these anti-caste movements like Lingayatism, etc.? Because for whatever reason (mostly geography and distance) Brahmins weren't able to socially and culturally dominate these places, which is why these places still speak Dravidian languages (or in the case of Maharashtra, have much less Steppe-related markers)

It's not racist or nazi to point this out. That's like saying it's racist to point out that the richest Mexicans are Spanish immigrants.

The commies of India don't hate brahmins or southern/Northern people , we hate the inherent caste structure based on land ownership and economic inequality

And wouldn't it make sense that said inequality is going to be worse wherever brahiminization was the highest?