It's really more of an epistemological statement. We can never know if the set of concepts available to us is sufficient to explain the reality we find ourselves in.
Epistemology has often been categorized under metaphysics, and rightly so, considering that the distinction between it and ontology already rests on a metaphysical distinction, and by no means the most likely one:
"The identification of will to power with interpretation and our exposition of Nietzsche's broad conception of interpretation allow us to answer the last of our initial questions: is will to power an epistemological or an ontological doctrine? The answer is that it is both, or neither: both, because it offers an account of knowing and being; neither, because it collapses the rigorous distinctions between subject and object, knower and known, upon which epistemology and ontology are traditionally founded." (Christoph Cox, Nietzsche: Naturalism and Interpretation, page 241.)
The only being we know from the inside is our own. And even in supposing that this is a matter of "we", that what we interpret as other human beings are essentially the same on the inside, we already make the postulation I spoke of. Now of course it's not necessarily the case that what we interpret as another being--my keyboard, for instance--is one being, and not rather (a part of) multiple beings; but to suppose that what it's like to be any non-human being is absolutely different from what it's like to be one of "us" is less parsimonious than to suppose that it's only relatively different, that it's essentially the same. Thus the interpretation of being as interpreting is the most reasonable interpretation--though only in the sense of
human reason, of course, and not of some supposed "universal reason". Remains the question as to the interpretation of interpretation--and with that, I conclude my post!