As for HBD, many HBDers subscribe to the theory of black athletic superiority even when its not true such as in the case of football and many other sports and seem to think intangibles have no effect on football performance beyond the QB position. "HBD" from my experience at least typically refers to a certain brand of human racial difference theory which is associated with the alt right but somewhat politically correct to be more appealing to the mainstream.
The said theory has a somewhat juvenile, simplistic view of racial differences at it's core in most cases from what I have seen, such as; blacks - athletically gifted and more masculine but cognitively/mentally inferior, whites - somewhere in the middle and asians - cognitively/mentally gifted but less masculine and athletically inferior, of course many of these claims are wrong/dubious/simplified. The mainstream doesn't like the idea of black/asian inferiorities in the aforementioned categories so whites are just disenfranchised in all aspects. The philosophy from what I have seen is that everyone is "equal" and has differences which balance out which I like to call the "aptitudes fallacy", it is more correct than the everyone is the same theory which was once pushed by the mainstream before (now they like to overtly push black athletic superiority for example) but still plenty wrong and probably not very different than the average persons' views on race.
Steve Sailer is someone who is known for his questionable pet theories in HBD and sports. I don't know how much he writes about it these days though.
As for HBD (as in human racial/ethnic differences in general) in the caste system, purported racial differences (which they have a poor understanding of) are often used to justify the caste system both by writers like Steve Sailer and the people behind the caste system, such as putting an emphasis on the importance of straight line speed but when all mental/physical factors are accounted for and performance observed the caste system begins to be exposed as not being meritocratic.