An element of a civil rights prosecution is that the deprivation is caused by an actor who is taking state action under color of authority; i.e., police officers.
It has been used against other than law enforcement officers but must always involve some sort of state action.
The DOJ may be considering just ignoring the state action requirement and hope the case is heard by a liberal judge who doesn’t care what the law is, or maybe trying to argue that Block Watch is the functional equivalent of a police department.