Let me paraphrase the sentence:
Even if there are metaphysical absolutes as we understand the term, our knowledge about them would still be human knowledge.
In this form, I think it is clear that this indeed is an epistemological argument. It is not a claim about metaphysics per se, it is a claim about what we can ultimately know regardless of the subject.
Now, many people object that this would state that human knowledge was (as a matter of fact) defective in some way, in the sense that we lack the cognitive abilities to fully understand ultimate Reality. Kant was a famous proponent of this view for religious reasons so that he had a place for a divine intellect in his system. But that is not the claim here.
Basically, the claim is rather about what Heidegger and Gadamer called the hermeneutic circle and Plessner, more broadly, the anthropological circle: Every reflection on existence is, by necessity, a reflection of human beings through a human mode of existence. This means that even though we have very good reasons to assume that our understanding of the world is broadly accurate and one way to map ultimate Reality (whatever that may be, given it exists), it still may well be that there are other layers and aspects which simply elude our (current?) abilities and way of understanding things. In the end, we only know of reality insofar as our existence can come in contact with it. Everything else is pure guesswork ("speculative metaphysics" is the technical term).
Think of it as the relation between a map and the actual terrain: no matter how good the map is, making claims about the map is very different from making claims about the terrain itself, even if you may accurately make claims about both judgeing only from the information of the map. Without knowing the terrain itself, the accuracy of your claims made on the basis of the map cannot be known, though. In a sense, we humans have our "human map" of reality only. Knowing this, absolute positive claims about metaphysics are necessarily dubious.
This is not to say that absolute metaphysical claims are wrong, nor that there are other forms of knowledge (ie. that there is any possible information beyond what "our map" already tells us) - that is why the claim is not self-refuting. It just says that a) strictly speaking, what we say about metaphysics is about how the world is to us, it is about "our" world, ie. we simply cannot know what it is like independent from our understanding, ie. in an absolute, objective way, and b) even if this kind of nitpicking is completely irrelevant for most of human lives and practices, it matters to keep that in mind when doing metaphysics. Doubting that the car rushing at you is real, or pondering ways in which coexistence of different corporal objects in the same space would be possible, does not exactly prolong your life, yet when you do philosophical metaphysics, dealing with skeptical objections is your bread and butter.
Long story short: It is an epistemological claim which bids not to pretend we had some divine ability to transcend human existence when it comes to metaphysics. This includes the possibility that this kind of (human) thought itself is unnecessarily cautious and everyday metaphysical claims are indeed spot-on, we simply cannot know: Who's to judge if not some form of intelligent being which has a 'higher' point of view from outside existence as we know it? And even if that existed (obviously in a way different from how we and the world do), it would obviously transcend our existence and thus wouldn't be able to tell us since otherwise, it would share existence with us which would, in turn, prevent it from having said higher standpoint....well...at least as far as we are able to think about these kinds of things anyway 😉
