This line does some work. On one level, it is red meat for colorblind white (and some non-white) liberals who require all black figures to be hopeful (I've discussed this more here: http://www.theatlantic.com/politics/archive/2015/08/between-the-world-and-me-book-club-not-trying-to-get-into-heaven/400271/).
On another level, it is doing some inter-group communication or what Stuart Hall called encoding/decoding and what Mark Anthony Neal translates into "black code" when he talks about Hall's work through modern media cultures. Obama is signaling here that she has noted those who have directed racist, sexist, classist rhetoric at her family. She has taken note.