If someone uses rot13, that is a highly informative. Is there any principled reason to like quoted words showing up, but not liking rot13? Anyhow, I think the disappeal of rot13 for TF-IDF is that it seems like a lower level feature than words. In particular, it is wasteful for it to show up more than once, if you're only doing top 11.
In some sense, I think the reason that the low level feature of rot13 is mixing with the high level feature of words is that you've jumped to the high level by fiat. Before looking a word frequency, you should look at letter frequency. With a sufficiently large corpus, rot13 should show up already there. I doubt that the corpus is big enough to detect the small usage by people here, but I think it might show up in bigrams or trigrams. I don't have a concrete suggestion, but when you look at bigrams, you should use both corpus bigrams and document letter frequencies to decide which document bigrams are surprising.
You've already surmised why rot13 words are undesirable. Just to check, are you suggesting I use n-gram frequency to identify rot13 words, or replace TF-IDF with some sort of n-gram frequency metric instead?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.