The two sentences have different meanings. "Interestingly cross posters" means that the posters are cross in an interesting manner; observing their crossness is interesting. "Interesting cross posters" means that the posters, who are cross-posting to more than one newsgroup, are interesting.
Laura used a play on words that obviously escaped you. As you continue to add comments, it becomes apparent that more escapes you than is caught by you. Your grasp is hampered by mental mittens. If you are going to play in this group, you have to step your game up. If you have a game.
Laura later amended her view to indicate that she was mistaken in her belief that you were interestingly cross. I agree. You are just run-of-the-mill surly. No one has suggested that you are interesting in any other aspect.
If the word "cross" is unfamiliar to you, information on the meaning is available in the dictionary.