First: It doesn’t seem to occur to Rosenbaum that there are black people who have no time for liberal guilt.
Second: You don’t have to believe in collective guilt to believe that institutions can unjustly act (or fail to act), and that this injustice can create a continuing responsibility for that institution to rectify the situation, and that this responsibility attaches to anyone in charge of that institution. (Which may be the voters.) Feelings of guilt are incidental.