I think the St Thomas reference comes from the teaching of his that there is and neither can be any particular attribute of God which is above or that plays disrepute to any other attribute of God.
St Thomas actually was building on a teaching of St Augustine when he said this. The idea being (from St Augustine) was that we as men cannot properly conceive of God because we have mortal, finite, fallen minds. Therefore when we read passages in Scripture like “God is love” or that God is wrathful, just, angry, etc, we are only able to conceive of this as it relates to us. Which is to say, we have never at any point loved the way God loves because have no perfect unstained impure love in us. Whatever we love, at least to some degree, we love at least a little bit selfishly. Therefore any love we have is also at least a little bit tainted by sin. This is an inescapable fact of the Fall. So instead of actually conceiving of God’s perfect love, we instead conceive of the best and highest most superlative love we can think of and try to understand how that would relate to us. The same is true for God’s wrath, mercy, long-suffering forgiveness, and even His anger.
Which of course is perfectly OK because when the Bible says God is love, the Apostle is trying to tell us something about God whom we cannot truly understand in a way that we can understand Him.
So that is Augustine.
Now to Thomas.
Thomas takes this idea, and thinks about these attribute of God and says all of God’s attributes must be perfect because God is perfect. Therefore nothing in God could ever be less than perfect else wise it could be said that God’s anger is perfect but His love is slightly less so. Or that His mercy is perfect but His justice is slightly more. Therefore any attribute of God must be perfect and in perfect harmony with all other attributes of God or else there would be division in God who is One. So His justice cannot work against His mercy, nor His love against His wrath, nor His forgiveness against His sinlessness, but rather all these attributes together must work in totality as One and perfect whole and never varying in degree and never working against each other.
How can this we may say? How can love not temper anger?
Thomas replies that we (again we see the strong influence of Augustine on Thomas’ thinking here), being selfish and prone to self worship above all other sin, MUST temper anger in order to forgive because we cannot truly ever forgive a slight for the sake of love. We are incapable of truly loving the one who has offended us because on some level we always will desire to seek our own. But God has no such flaws. And so the Perfect is able to love and be wrathful and forgive and be just because He in not a man and neither is He as a man.
Therefore, Thomas argues, any of God’s attributes are rightly called by God’s Name. So it not only that God is love, but also God is wrath, God is mercy, God is justice.
I think that may be what Hahn in driving at.
BTW the Augustine teaching can be found in De Doctrina Christiana and of course Thomas’ in the Summa Theologica, but I got this particular explanation from Kreeft’s Summa of the Summa.