Parent is probably using murder rates because murder is very unlikely to be unevenly enforced, with the implicit implication that these differences extend to other violent crimes (but the data there is muddied by differential enforcement).

Here is the strongest presentation of this argument that I've seen: https://randomcriticalanalysis.com/2015/11/16/racial-differe...

I don't have a well-formed opinion here, but IMO it's worth considering many possible combinations of factors.