This is consistent with what I had believed. Of course St. Thomas Aquinas wrote before the Council of Florence and it is a difficult subject, but it does look as though he made a mistake here.
I believe the Bull and Aquinas are saying the same thing – but it’s not that easy to grasp it in Aquinas. This paragraph in the Summa link is where it can be most easily (if I can use that word about Aquinas

) seen. I’ll color code the sections from the Bull and Aquinas to show where they are saying the same thing.
Aquinas
We must say, therefore, that grace was bestowed in circumcision as to all the effects of grace, but not as in Baptism.** Because in Baptism grace is bestowed by the very power of Baptism itself,** which **power Baptism has as the instrument **of Christ’s Passion already consummated. Whereas
circumcision bestowed grace, inasmuch as it was a sign of faith in Christ’s future Passion: so that the man who was circumcised, professed to embrace that faith; whether, being an adult, he made profession for himself, or, being a child, someone else made profession for him. Hence, too, the Apostle says (Romans 4:11), that Abraham “received the sign of circumcision, a seal of the justice of the faith”: because, to wit, justice was of faith signified: not of circumcision signifying. And since Baptism operates instrumentally by the power of Christ’s Passion, whereas circumcision does not, therefore Baptism imprints a character that incorporates man in Christ, and bestows grace more copiously than does circumcision; since greater is the effect of a thing already present, than of the hope thereof.
The Council of Florence, Bull “Exultate Deo,” 1439.…There are seven sacraments of the new Law: …, which differ a great deal from the sacraments of the Old Law. For those of the
Old Law did not effect grace, but only pronounced that it should be given through the passion of Christ; these
sacraments of ours contain grace, and confer it upon those who receive them worthily. …
Baptism is an actual instrument which contains the grace;
Circumcision is only a sign. It was because of their faith that God conferred the grace which Jesus would merit.