Well, if the terra incogntio has any relationship at all to the thoughts you do understand, such that the terra could be recognized as a part of or related to a cognitive state, then the terra is going to consist in stuff which bears inferential relations to what you do understand. These are relations you can necessarily traverse if the alien can traverse them. Add to that the fact that you've already assumed that the aliens largely share your world, that their beliefs are largely true, and that they are largely rational, and it becomes hard to see how you could justify the assertion at the top of your last post.
And that assertion has, thus far, gone undefended.
Well, I justify it by virtue of believing that my brain isn't some kind of abstract general-purpose thought-having or inferential-relationship-traversing device; it is a specific bit of machinery that evolved to perform specific functions in a particular environment, just like my digestive system, and I find it no more plausible that I can necessarily traverse an inferential relationship that an alien mind can traverse than that I can necessarily extract nutrients from a food source that an alien digestive system can digest.
How do you justify your assertion that I can necessarily traverse an inferential relationship if an alien mind is capable of traversing it?
Here's the new thread for posting quotes, with the usual rules: