Yes, I saw that warning as well. That, to me, implies that the product cannot stand up to near continual water contact. Being as its an exterior product, it would likely hold up in a 1 or 2 day constant rain??
Above, mark poulson mentioned that he uses that same company's interior grade product without problems. I believe there are others who have used "non-waterproof" products successfully as well.
Like we all are, I'm just trying to find the grail of topcoats. Still love to hear anyone's input. Perhaps I will be the guinea pig on this one?!?