What each apologist believed individually was never the doctrine of the Church, so if Cyril believed scripture (which was not even canonized at that time) was the sole source of authority, that was not the opinion of the rest of the Church. What the Church believes and sets as dogma is what the Church as a whole (consensus patrum) declares at a General (Ecumenical) Council.
So, the short answer to his drawn-out question is: yes, he did leave the Holy Catholic and Apostolic Church!
Correct. To top it all off, the Author (webster) must not have read much Blessed Saint Cyril,because he was NOT a solo scripturalist private interpretation believer who taught outside what the Church believed Scripture to say.
Example
"But in learning the Faith and in professing it, acquire and keep that only, which is now delivered to thee by the Church"-Cyril of Jerusalem, Catechetical Lectures, 5:12 (A.D. 350).
I have run across this William Webster heretic before when he tried using the Church Fathers to deny the real presence of Christ in the Eucharist. He did not fair too well either
Webster is just another charlton lining his pockets with money and tickling ears teaching against historical Christianity by twisting what the ECF's said in his own self interpretation that is not consistent to the ECF's writings as a whole