I wouldn't be too surprised if the hypothesis is true for unmodified humans, but for systems in general I expect it to be untrue. Whatever 'understanding' is, the diagonal lemma should be able to find a fixed point for it (or at the very least, an arbitrarily close approximation) - it would be very surprising if it didn't hold. Quines are just an instance of this general principle that you can actually play with and poke around and see how they work - which helps demystify the core idea and gives you a picture of how this could be possible.