God has really revealed to us that He is one ousia with three hypostases.
Surely this is the human formalisation of the revelatory data, and to say that the theory is transparent to the data is surely to put the cart before the horse. Augustine, for example, is extremely cautious about the language he uses, clarifying at length that this language of the trinity is a fallible human construct and falls short of the reality:
They indeed use also the word hypostasis; but they intend to put a difference, I know not what, between οὐσία and hypostasis: so that most of ourselves who treat these things in the Greek language, are accustomed to say, μίαν οὐσίαν, τρεῖς ὑποστάσεις or in Latin, one essence, three substances. But because with us the usage has already obtained, that by essence we understand the same thing which is understood by substance; we do not dare to say one essence, three substances, but one essence or substance and three persons: as many writers in Latin, who treat of these things, and are of authority, have said, in that they could not find any other more suitable way by which to enunciate in words that which they understood without words. For, in truth, as the Father is not the Son, and the Son is not the Father, and that Holy Spirit who is also called the gift of God is neither the Father nor the Son, certainly they are three. And so it is said plurally, I and my Father are one. For He has not said, is one, as the Sabellians say; but, are one. Yet, when the question is asked, What three? human language labors altogether under great poverty of speech. The answer, however, is given, three persons, not that it might be [completely] spoken, but that it might not be left [wholly] unspoken. (De trin. 5.8-9.10)
It is also unfair to Ps.-Dionysius to suggest that he affirms that we can't know things about God in a sense that is undermined by revelation. Rather he straightforwardly affirms that he discusses in what sense god is three and one in a now lost tract. Similarly, one of the points of the Divine Names is to affirm that we can say these things intelligibly of God: "We then, having collected these intelligible Divine Names, have unfolded them to the best of our ability". (The Divine Names, 13.4) But rather he is speaking to the tension, itself noted by plenty of other orthodox authors, between the expressible and inexpressible and the poverty of our language, even if correct, to delimit the divine substance. As such, like Augustine, he affirms the poverty of the trinitarian language that he suggests we should beyond through unknowing.