Interestingly I had the same experience going on a vacation to the Balkans. To an outsider, Croat/Serb/Bosnian are all basically the same - your languages are mutually intelligible, your food is all tasty meat cuts and flatbread, you look more or less the same. But apparently within this region, this is a life-or-death distinction.
https://en.wikipedia.org/wiki/Narcissism_of_small_difference...
But also, "castes aren't based on anything real and are obvious bullshit" (paraphrasing the original comment) is not going to be obvious to everyone and not a very helpful statement. Kind of like saying "the world really isn't flat" -- we know, but it's not gonna help anyone because you're not going to stop flat earthers from existing or being sincere about their beliefs.
I’m not sure that this is as insightful of a post as you envisioned.
https://www.bloomberg.com/letters/2021-03-18/caste-based-dis...
It may be.
But there are enough examples in history to understand that this may not be a trivial matter, either. Could you tell if someone is Jewish or not? Could you tell utus and tutsis apart?
But I've worked with ~100 Indian co-workers and several were very vocal about their caste because they were from the "higher" caste; well-educated, children of politicians and scientists and vegetarian.
I think it matters if it's causing discrimination at my workplace.