True enough. It belongs in the same category with omnipotence, omniscience, perfection and other examples of the infinite which do not exist in reality.
Indeed! But then what, do we throw all of this out? Is something only useful or relevant if it is not factual and tangible, absolute and unrefutable? Could these concepts not act as catalyzers for the development and expansion of our individual experience?
So, even if we were to consider that the only Nirvana worth talking about needs to be the 100% utopic and idealized version of itself, we could still definitely derive benefit from it, even if it were false (unless, of course, the only reason to expand our intellect were to check off the magical list of life which things are real and which ones are not, turning our backs to anything that cannot bite us, be eaten, or copulated with).
Having made that point, our conceptions of what is factual or not is extremely limited to our own experience, so if you haven't ever experienced something that surprised you and drastically altered your perspective and self, then it makes sense that anything you haven't digested yet would seem absolutely indigestible, and anyone who consumes that thing would seem crazy.