There is nothing saying Nobodies didn't exist in BBS. Obviously heartless did to some extent therefore nobodies probably did to, though they were most likely very few at the time. Heck, it's possible MX created a Nobody when he released his heart at the end of Terra's story, we just haven't seen it yet.
As for why MX put the nobody symbol on his armor, well who knows. Perhaps the symbol originaly did not represent the nobodies and it was actually a symbol MX created.
nomura already said in an interview that xemnas was the first nobody, MX didnt create a nobody when he released his heart because he didnt become a heartless.