Yancey’s information is not 100% accurate. Aquinas and Duns Scotus never knew one another, in fact, Duns Scotus would have been 8 years old when Aquinas died.
The fact is that Aquinas did not take a strong stance on the subject. In fact he wrote that “this is not a very important question” given the actual economy of grace and he himself admits that the opposite “opinion can also be called probable”. His stance in his Summa theologica is that “it is more probable” that “the work of the Incarnation was ordained by God as a remedy for sin, so that, if sin had not existed, the Incarnation would not have been.”
When Duns Scotus came on the scene and said that it was “absurd” to say that God’s greatest work in all creation (the Incarnation) was “occasioned” by sin, the disciples of Aquinas then took a strong, definitive stance based on what Aquinas called “more probable” (for the thomists or disciples of Aquinas it was not more probable, but quasi-infallible dogma—makes for good debates :)
>> Duns Scotus <<
No-one ever said SCOTUS was infallible. Even though liberals treat certain justices as if they were.