Pertinent quote from the results section of the publication:
Most people who come from a socio-economically less favorable background do not commit more crime than people who come from a more favorable background, and it also happens that people from a more favorable background do commit crime. This means that even if there is a connection between socio-economic background and involvement in crime, that connection is weak. It is not possible to appreciably predict who will commit crimes based on knowledge of people's socio-economic background.
https://bra.se/publikationer/arkiv/publikationer/2023-03-01-socioekonomisk-bakgrund-och-brott.html
Both factors correlating to a rise in crime sounds perfectly consistent to me. If you imagine that black and white aggregate IQ scores are both represented by bell curves, with one lower than the other, it's reasonable that the lower group will be more negatively affected by weakening cultural traditions that made up for its deficits. The higher group will also be weakened, but less so. One community becoming "somewhat functional" in fatherless homes, the other being utterly destroyed.
A more important question is how much stronger would white families be today if they hadn't been scattered into isolated individuals, with the men made emasculated and subservient to women's concerns?