More than anything it just seems like a useless gesture? I can't really comment on the notion of black-only/nonwhite spaces; I'm inclined to think that their benefits outweigh whatever issues they might pose to class solidarity across race but I haven't read anything on it. But Twitter is not really a platform that's conducive to carving out spaces like that. You kinda just have to deal with whatever mfs decide to get in your replies and mentions.