1. Genetics, upbringing and culture all matter
2. Diversity within an ethnic group is extremely large - often similar magnitude to diversity between ethnic groups
3. Therefore, to judge someone on their ethnic average will be inaccurate at best, offensive at worst
I think that this position has been misconstrued by pop-right / alt-right figures to create a false narrative in the culture wars.
Most scientific research into eugenics, "racial differences" and such is motivated by reactionary value systems. Most of it can also easily be classified as pseudoscience and is published in fringe journals (e.g. see Emil Kirkegaard's "research" and his "journals").
Since this topic is so overrun with reactionaries trying to scientifically justify their views, it's extremely risky for a serious journal to publish research into it and for institutions to fund this research in the first place. Because in all likelihood that reasonable accredited scientist prompting you about wanting to do the research is either personally pursuing such ideological goals or representing the interests of such a group.
In other words, because of all the incredibly unscientific "research" actual racists and eugenicists have done in this field before, it's extremely hard to do any work in this field without either risking being perceived as supporting their views (by opponents) or as validating their unscientific prior work (by supporters).
It's a bit like debating the Israel-Palestine conflict: if you criticize Israel for their actions in Palestine, you might have genuine human rights concerns or you might just do so as a proxy for anti-Semitism. If the people doing the latter wouldn't exist (and there wasn't such an obvious historical precedent for why they're scary) it wouldn't be such a hot button issue. Go ask a leftist how this usually plays out for them.