What they said was "interracial couples demonstrated a higher level of mutual IPV than monoracial white couples but a level similar to monoracial black couples".
In other words, inter-racial couples see a violence level similar to black couples. And black domestic violence level is high. While it's hard to get the federal data, in Oklahoma, for example, blacks committed 23% of domestic homicides while being 7.4% of the state's population
(Adding): it’s not surprising that the inter-racial homicide rate is very close to the black-couple rate, since most inter-racial couples seem to be white-woman/black-man, and the male commits 77% of domestic homicides (according to the first link in my prior post)