"The Protestant appeals to his own (sometimes heretical) interpretation of Scripture, which for him trumps any apostolic tradition of the Church; thumbing his nose even at (Craig’s phrase) “Nicene orthodoxy.” It’s a prime example of how sola Scriptura can lead to heresy (has no final answer against it): even very serious heresy involving the theology of God: theology proper, and Christology. This is what can and does happen by denying the infallibility of the Church and ecumenical councils."
The irony of Dave Armstrong's reasoning is that he himself appeals to the Scriptures in refuting this christological heresy:
"Jesus distinguishes His human will from His Divine Will, but completely subordinates the former to the latter. According to Ludwig Ott in Fundamentals of Catholic Dogma (p. 148), here are the scriptural proofs of the orthodox Catholic (and mainstream Protestant) position (with one or two of my own added) [Matthew 26:39; Luke 22:42; John 5:30; 6:38; John 4:34; 5:19; 8:29; 14:31; Romans 5:19; Philippians 2:8; Hebrews 10:9; John 10:18; Matthew 7:21; 12:50; 18:14; John 6:39-40; etc.]."
If the principle of Sola Scriptura is inadequate to address Monothelitism, then what purpose or reason is there for this Roman Catholic apologist to use Scripture to refute this theology? He is, in essence, contradicting himself. His argument is self-refuting.
The only way Scripture alone would be insufficient to refute Monothelitism is if it had nothing to say on that subject at all. However, plenty of arguments against this doctrine can be made from Scripture itself and thus it is sufficient to address heresy.