The rationale for McGrath's typical incredulity evapoates if you consider that Luke is actively hostile to his sources, and that the redactional activity of the evangelists in general was not characterized by a desire to avoid contradiction, but precisely to contradict where they disagreed with their predecessors' presentation. Harmonization was a later concern. It seems clear to me that Matthew was intended to supplant Mark, and I think the author of Luke had the same idea.