McGrath doesn't quite put it this way, but what emerges from his broad-stroke depiction of the various reformational movements is that Wittenberg was unique in placing the theological reform of the Church ahead of the humanistic reform of manners. To be sure, Wittenberg co-opted Humanism, just as Luther was, at least early on, co-opted by the Humanists (Erasmus cheered from the sidelines until his disastrous defense of a free will overagainst God in his Diatribe, and Luther's subsequent, wholesale, and irrefutable articulation of biblical teaching on the freedom of the will, or lack thereof, in his 1526 De servo arbitrio).
The De servo/libero arbitrio incident itself demonstrates the difference between humanistic reform that also concerned itself with theology, on the one hand, and theological reform that used the tools of Humanism, on the other. For Erasmus, the center of the issue in the debate over the will was human manners: how, he reasoned, if humans have no free will, can we Northern Europeans emerge from the pigsty that is our life? For Erasmus, Scripture is subsumed under humanistic tenets.
For Luther, on the other hand, the question in front of him had nothing to do with human manners, which before God (coram deo) were, even at their best, filthy rags, even if before men (coram hominibus) some degree of righteousness could be ascribed to them. But here's the key: in the argument put forth by Luther in De servo arbitrio, he uses the tools of Humanism (rejection and refutation of scholastic argumentation, for example; close examination of the primary text, Scripture, even, albeit citing largely the Vulgate, with reference to the thought underlying the Latin expression based upon original Hebrew and Greek; use of typical humanistic tropes, such as the author's professed lack of eloquence [on which see the many Wittenberg declamationes, Ciceronian and learned in the highest degree, but accompanied by the disavowal of any real skill on the declaimer's part]), but Scripture is the driver of the argument.
This distinction between Wittenberg thinking and the thinking of other reformational movements is fundamental. The driver of Huldrych Zwingli's Zürich Reformation was humanistic: humanism shaped his critique of contemporary life, the way he read Scripture, and the ends to which he engaged in churchly reform, which was, in the end, a reform of manners in whose pursuit theology was enlisted.
Here again the genius of the Lutheran Reformation shines through: even with the darling of the Renaissance and Christian Humanism, Philippus Melanchthon, quite literally on campus at Wittenberg, Humanism was kept safely where it belongs: in the left-hand realm of God. To be sure, Humanism did aid in the pursuit of the theological aims and goals of the Wittenberg Reformation, but it did not define them. Much as the two realms of God, the left and right, resist any sort of finalizing separation of one from the other on this side of the eschaton (in my role as Hausvater in our home, I participate simultaneously in both realms, just as I am, paradoxically, also saint and sinner at one and the same time), so also was there an interpenetration in Wittenberg of Humanism and Christian theology. But there was never any doubt which was Queen.