St Thomas Aquinas teaches that Christ, the man, was not subject to concupiscence and the inclination to sin which is the revolt of our lower nature and passions against our higher and spiritual nature of intellect and will. Concupiscence is the result of original sin, it is one of the effects we experience in ourselves due to original sin as St Paul says: “For the flesh has desires against the Spirit, and the Spirit against the flesh; these are opposed to each other, so that you may not do what you want” (Galatians 5: 17). But Christ is without sin including original sin. Further, the soul of Christ was full of grace as St John says:
“And the Word became flesh*
and made his dwelling among us,
and we saw his glory,
the glory as of the Father’s only Son,
full of grace and truth” (John 1: 14).
Being full of grace is incompatible with sin or the inclination to sin which is concupiscence. The soul of Christ was also filled with the Holy Spirit. Further, Aquinas says that Christ, the man, had all the virtues including the moral virtues most perfectly. God created Adam and Eve in original holiness and justice which means their lower nature was subject to their higher spiritual nature, they were in harmony with themselves, God, and the rest of creation. Before the fall, Adam and Eve did not experience concupiscence which is an effect of the fall. Now, I don’t think we can admit that Christ, the God-man, the new Adam, was incarnated with a less perfect soul or one infused with less grace than when God created Adam and Eve. However, we do admit that the human nature of Christ did experience other effects of original sin such as being tired, suffering, and of course, death. For Christ assumed a fallen human nature like us in order to heal it. But not a fallen nature in all its effects; for Christ was not conceived with original sin on his soul nor did he experience, as Aquinas says, concupiscence or the inclination to sin, the revolt of the lower passions against the spirit. So, what the author says in the quote you give, namely, “initial (unchosen) stimulous,” which is concupiscence, I think is an error. Aquinas says “although He [Christ] suffered no internal assault on the part of the fomes [concupiscence] of sin, He sustained an external assault on the part of the world and the devil and won the crown of victory by overcoming them.”