If you pay any attention to the video game industry at all, you know that the ESRB is always pushing the game companies, to put one of their ratings on their games. I kinda doubt they would look to sue anyone for putting such a label on an item, as that would defeat the purpose, and the reason of their existence at all.
They would sue or at least complain because they haven't actually reviewed the game. But this is a pretty dumb thing to argue over.