His point is crystal clear: he's saying you can't claim to rigorously study a relationship between race and crime by relying solely on conviction data, because racial prejudice and its legacy (for instance: what neighborhoods different races tend to live in, and whether those neighborhoods are enforcement priorities) has a profound impact on criminal conviction.
But I could also argue that not all crimes are caught in the data, because the law is written wrong. In fact, I could argue the law as it is written is thus criminal. After all, this is a democracy and the people are responsible for the laws. The people are incapable of drafting the laws in a non discriminatory manner (because racial prejudice and its legacy). Or, I could argue the exact opposite. Both are equaly weak/strong. But, this style of debate is incoherent. For it to be coherent, it would require special knowledge. And is otherwise a variant of https://en.wikipedia.org/wiki/Special_pleading
