Alfaric does an interesting observation: it is impossible that in the same moment 12 apostles would have taken the pen and written the same identical things.
So the title "memories of 12 apostles" is justified by the fact that, just as, according to the legend, 70 rabbis wrote the Septuaginta out of inspiration by the same spirit, so the new torah for Christian use was written by the 12 apostles in the same moment under the same divine inspiration.
Alfaric thinks that a such gospel had to precede all the other gospels, since it only went against the tendance to attribute a Gospel to X, Y, Z.
Matthew, who was based on the "Gospel of 12 apostles", realized the anomaly and individualized the author, from 12 to only 1, precisely "Matthew" (meaning: Disciple).