“One attempt to reconcile the Gnostic doctrine [of the unreality of evilness] of matter with the apostolic teaching about Christ was the theory that the body which our Lord took at His coming into the world was not a real body but a phantom one. He only seemed to inhabit a material body, and from the Greek word dokein ["to seem"], people who held this theory were known as Docetists. But if Christ's incarnation was unreal, His death and resurrection were also unreal; and the whole gospel message was thus evacuated of its truth and power: one unhappy legacy of this short-lived phase of Christian heresy remains to bedevil Christian witness to Muslims up to the present day. For when the Koran says of Jesus that "they did not kill Him, nor did they crucify Him, but they thought they did", we may infer that Muhammad was indebted for this idea to a Christian source tainted with Docetism.”
http://www.worldofquotes.com/author/F.-F.-Bruce/1/
Each of these had a slightly different telling of what they thought Christ's nature was. And Arabia had the heresies that had been proven wrong elsewhere, like the Gnostics, Arians, etc.
Remember also that Medinah was a Jewish city and Mo took a lot of the beliefs of the Jews and twisted them (like having Abraham sacrifice Ishmael instead of Isiah etc) and Mo himself was illiterate, hence his retelling of the nativity in the Koran reads like the birth described in Revelation.
Finally, as you correctly point out, the concept of Jesus being a phantom also slips in and is also contradictory with another part of the Koran/hadiths that states that Christ was "replaced" on the cross and laughed at those trying to kill him..