No, it is an attack on the people who claim to be for social justice, and yet let comments like the one by this man slide. If my father ever offered violence in response to insults hurled in my direction, I would be appalled for several reasons:
1) Physical violence is an inappropriate response to this sort of taunt. There are many better ways to handle trolling by idiots on the Internet.
2) My father raised me to be capable not only of fighting my own battles, but to be capable of determining which battles are worth fighting, and which ones are simply beneath even contempt, and should be given the consideration they are due. I.e., none.
3) The fact that the label "slut" was the one that enraged this man. Personally, I do not feel that consensual sexual behavior is anything to be ashamed over. I would be more amused by being labeled a slut, much the way I would be amused by being labeled a lesbian. As a feminist myself, I think the better way to handle slurs of this nature are to essentially take them back and remove their stigma, rather than offering to physically harm the person making the slur.
But hey, I am just a regular, garden-variety "atheist", what do I know about social justice?