Me too. Having worked professionally in communications fields over a long time span I've often been glad I had his warnings. It saved much grief, knowing in advance that using the word "tree," for instance, doesn't mean much to anyone else unless I specify the type of tree, size, location, whether it's spring, summer, fall or winter in the case of deciduous species, etc. Language is very imprecise, and even specifying all those things people always add their own subjective connotations as you point out -- just read FR for plentiful examples, LOL.
Unlike some other posters to this thread, though, I think it's important to try to decipher as much meaning as possible from past cultures, including whatever can be learned from quipu. We should never be so arrogant in our modernity (or is it post-modernity?) as to think our predecessors can't teach us anything. Ultimately you're right, of course: we'll never completely understand any historical communications, even our own, in their exact cultural context.
I find trying to decipher ancient cultures one of the ways out of the imprecision Korzybski warns about. As resourceful as humans are, it never ceases to amaze me that scholars are actually able to advance at all in understanding ancient cultures... i think it gets down to the nitty gritty of what really distinguishes this species, its ability to imagine and hence create a context, however tentative, in ones mind, of something that is either long ago or far away...