Just to add to Brammer's reply, my understanding from the information supplied by LJ in their post is that the crossposting option apparently OVERRULES any flocking or privacy settings on the journal / entry being commented on (including screening). So regardless of being flocked, someone commenting to your post can crosspost their comment (and any quotes etc) and the post will show the links to your journal. It won't open up the rest of the post, but it does make that comment and all its detail public.

If I have this wrong in any way, hopefully someone else will clarify, but this is what I understand.