Disparate Impact is not a real explanation, because it's equivalently useful at any stage of the argument. For example, "Disparate Impact" might explain why banks don't lend so much to group X. So if we find some good underlying variable that explains why banks would be disproportionately unlikely to lend to group X, "Disparate Impact" now explains that factor. And when that factor gets explained, it explains the next factor.

Disparate Impact is just "God of the Gaps" for discrimination.