When they arrived at the Rosh Hashanah observance they could not retell the baptism story, which they had already used. So what did they do? They reintroduced John the Baptist into their texts at exactly the Rosh Hashanah moment by using the technique of a flashback in which John reappears, not in person, but in the voice of messengers sent from John. The story is essentially the same in both gospels and, in my opinion, constitutes another hint that Luke had Matthew to draw on as well as Mark.
