(Gospel of the Twelwe Holy Apostles, syriac version)
Compare that incipit with Mark 1:1-3:
2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee.
3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
4 John did baptize in the wilderness
Do you note the difference? In the first incipit the "angel" is Jesus. In the Mark's incipit the "angel" (angelos) is euhemerized as a human "messenger".
But then we remember about Exodus 23:20 et seq.:
We realize that the "angel" in question is not only a divine messanger as opposed to a human prophet à la John-Baptist, but he is just the angel Joshua (!!!) who conquered Canaan going ahead of the Israel's armies.
The rapid inference is that John the Baptist was inserted to eclipse the figure of the previous "angel" who had to come. I can only think that the modification was made because the previous "angel" was too much mythical as figure, even for these writers!