The only rule that applies to the LTD thread that doesn't apply to other threads is that other forums can't be named at all, which was instituted because discussion of other boards was pretty much the only thing that was going on in the previous LTD towards the close of its existence. Other than that, every other rule applies to the rest of the board as well; the LTD is simply more strictly moderated. I would say the LTD is mostly dead compared to its previous incarnation because the people who previously kept it active were more interested in discussing what people on other forums were saying than they were in actually discussing the LTD. Also, there simply isn't much left to discuss. Nearly everyone already knows what everyone else's opinion is, and they've been discussing it for five years or more in many cases.
Disallowing discussion of other boards is far from unheard of; it is hardly unheard of for people, coming to a new community, to wish for a fresh start, and it is not unreasonable to accommodate that desire.