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After a careful re-reading and actually looking at the footnotes of the work I found the answer myself. St John combated a developed form of Pelagianism put forth by Leporius in his own area:

Leporius was apparently a native of Treves who propagated Pelagian views in Gaul, ascribing his virtues to his own free will and his own strength; and going to far greater lengths than his master in that he connected this doctrine of human sufficiency with heretical views on the Incarnation; thus combining Pelagianism with what was practically Nestorianism, teaching that Jesus was a mere man who had used His free will so well as to have lived without sin, and had only been made Christ in virtue of His Baptism.

This footnote is on pg 552 of the second series of the Nicene-Post Nicene Fathers (same reference as the question).

So Pelagius himself did not ascribe to these views, but the brand of Pelagianism that St. John dealt with seems to have taught this. St. John goes on in the same chapter to mention how Nestorius's own writings betray a sympathy for the views of Pelagius.

It seems to be for this reason that St. John views the Pelagian and Nestorian heresies as linked, where one will naturally lead into the other after a period of time, because if you isolate the human from the divine you inevitably start postulating that man can operate by his own power alone to attain godliness.

It is worthwhile to note that Leporius's letter of repentance and rejection of Pelagianism is included in part on the next page in which Leporius makes no mention of free-will or God's sovereignty, but only talks about the Incarnation:

Therefore the God-man, Jesus Christ, the Son of God, is truly born for us of the Holy Ghost and the ever-virgin Mary. And so in the two natures the Word and Flesh become one, so that while each substance continues naturally perfect in itself, what is Devine imparteth without suffering any loss, to the humanity, and what is human participates in the Divine"

I think that perhaps this view was what St. John viewed as foundational to our understanding of salvation from which implicit rebukes of Pelagianism, Nestorianism, and I would think also Augustine's more extreme views can be drawn.