The key phrase is "it is reasonable to presume". This is not a peculiar maneuver. Historians are constantly presuming something because they see the presumption to be reasonable.Since the canvas is drawn, and since several titles given to Jesus in the fourth gospel also appear in the synoptics, it is reasonable to presume that what we see in John are numerous segments of common gospel tradition already available to John in written form when John determined to write his gospel.
Two Questions:
1) Suppose I told you that on this presumption the scholar launches a devastating attack against the Christian case? Would you say his presumption is reasonable? Why, why not?
2) Suppose I told you the author were a Christian, and launched a defense for Christianity based on this presumption. Would you say the presumption is unreasonable? Why, why not?