Did Pelagius or Pelagians in General Deny Orthodox Christology In St. John Cassian's work "On the Incarnation: Against Nestorius" he claims that Pelagius believed


  that Jesus Christ had lived as a mere man without any stain of sin, they actually went so far as to declare that men could also be without sin if they liked. ... They added as well that our Lord and Saviour became the Christ after His Baptism, and God after His Resurrection."


On the Incarnation of the Lord Book I Chapter III. Page 552 or 553 of the second series of Nicene and Post Nicene Fathers.

The author of this answer (https://christianity.stackexchange.com/a/52958/27623) takes issue with that claim. Is this actually what Pelagius himself, or Pelagians in general believed? 

It seems logical that the patristic teaching of the incarnation implicitly teaches an indwelling of grace in mankind brought about by the union of God and Man within the person of Jesus Christ and therefore this is something that the Pelagians want to deny, so they explain that Jesus Christ was simply a man that through his own will and efforts obtained the indwelling of God (sort of like a Christian version of Nietzsche's super-man) and is gracious to us by giving an example.
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 Divine 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.