What makes it significant is the size of the disparity (and how much larger it is than the statistical crime disparity). What my belief is based on is many things, but chiefly the history of the mistreatment of black people in the US (and specifically the history of mistreatment by police), and the opinions of black people today.
Even these things together don’t mean that I’m absolutely certain that the 21 times is explained by racism, but I suspect that it is involved. Further, since many or most police departments have been reticent to keep or share relevant data, I suspect their unwillingness may be at least partially be based on their worries that there might be actual problems in the data. But I won’t conclude anything without having all the facts – so for now, I just suspect.
Historically, when there was some significant disparate treatment of black people in America, picking “racism” as one of the major causes was almost guaranteed to be correct.
There are other statistical disparities (besides that for shootings) that may show possible mistreatment/unfair treatment of black people (all black people, not just young men) by police.
No such reasons have been offered that specifically explain why 21 times is justified. It’s been offered that police may feel more fear (not that I believe this justifies shooting more people), but this doesn’t explain any particular value, and therefore doesn’t help us here.
Further, there are many types of bigotry – someone (or some institution) can be bigoted or racist against, for example, young black men, while not being bigoted (or racist) against black women or older black people. Considering that most of the historical racist tropes in America have been directed against adult (and usually young-ish) black men (that they were a danger to white women, that they were aggressive, violent, animalistic, and unable to control themselves, among many other racist tropes), this may not even be particularly surprising.