As I remember the argument, one of he methodological principles for historical validity is embarrassment. If all the gospels devote time and effort to clarifying how much greater is Jesus compared to John, one can reasonable conclude that that was a delicate issue because Jesus was closely connected with John, being baptized by him and drawing disciples from his circle. There must have been cause for some embarrassment, which is best explained by Jesus being  within the John's circle as his follower, before leaving the wilderness to announce the gospel in the towns and villages. I think the synoptic gospels are like the lives of eminent persons, political (Alexander) or philosophic (Diogenes Laertius' Lives), meant as models for imitation, filled stories and wonders, yet not without some grounding in history presently ascertainable. You don't get much and even what is real history is subject only to historical judgment.