As I understand it "nerdfighter" is just a name for a member of that specific fandom, and defining them by the fandom's charity work is mostly just the brothers being very grateful for having a fandom and especially one that does a lot of charity work (esp. when they mobilize them to do so).

I often think of them (more the brothers than the fandom which I don't interact much with) as being my-tribe, and exemplary members of such, but not as my-tribe as those in my specific niche of nerddom (i.e. the LW sphere), and sometimes annoyingly mainstream because of that.