It appears that whites don't care when other whites have friendships and support networks with blacks. This allows whites to have broader support networks without undermining their in-group solidarity. Blacks apparently behave differently, imposing a penalty on their racial fellows for having friends outside their race. An indifference to racial solidarity and discipline is an economic advantage.
The abstract doesn't cover why this broad racial advantage for whites exists and what happens when you throw asians in the mix.
From a political standpoint, Democrats identifying the GOP as the "white people's party" may be tapping into this racial discipline effect in ways that have not been studied up to this point. It would be interesting to see if there's a liberal/conservative divide in the racial solidarity dropoff of having white liberal vs white conservative friends.