First let me pick up again on the authors' suggestion "that when Matthew and Luke make editorial changes to Mark, none of Matthew's changes show up in Luke and vice versa." They add that "If Matthew had known Luke -- or if Luke had known Matthew -- then surely some of the changes would be apparent" (116). It's a remarkable statement given that the triple tradition in fact features hundreds and hundreds of agreements between Matthew and Luke against Mark. It is true that Two-Source Theorists have explanations for these, some of the major ones as "Mark-Q overlaps" and some of the minor ones as due to independent redaction, or textual corruption, but denying that they are present ought not to be a serious option in the discussion.
Other statements that I would like to question relate to their brief arguments for Marcan Priority, e.g. the statement that "While Mark's gospel is shorter, each of Mark's narratives is longer, and often details are removed in Matthew (but still assumed)" (115). I like the drift of the latter element here, which coheres with my argument from Fatigue, but the statement that "each of Mark's narratives is longer" is not true at all. Many of Matthew's parallels to Marcan narrative pericopes are longer. The impression of consistent Matthean brevity is derived largely from the triple tradition material in Matt. 8-9, where Matthew is shorter in his parallels with Mark. Elsewhere, this is less often the norm.
A third puzzling claim is the following:
In Mark 6.5 we learn that Jesus "could not do any miracles [in Nazareth]." Matthew appears to supplement this potentially embarrassing admission by saying Jesus did no mighty work there "because of their lack of faith" (Matt. 13.58). (115).The line quoted from Matthew is identical in Mark 6.6; Matthew does not so much supplement Mark here as contract it. I think the authors are aiming to make a contrast between Mark's "could do no . . ." and Matthew's "did not . . .", which is the way that this argument is usually set up, but the re-statement of it loses the necessary clarity to make that point (and, incidentally, it's worth taking a look at Peter Head's Christology and the Synoptic Problem for a nuanced discussion of this example).
A fourth issue is a diagram that appears on the bottom of p. 115. The diagram has arrows illustrating Marcan Priority plus Luke's use of Matthew. In other words, it appears to be an inadvertent and unlabelled diagram of the Farrer theory. But it appears to be used, in context, as a diagram that is supposed (just) to illustrate Marcan Priority, which is what is being discussed either side of it. It may be that the arrow pointing from Matthew to Luke is deliberate, and that a subtle allusion to the Farrer theory is intended, but I doubt it. The text goes on to talk about "whether Matthew used Luke or Luke used Matthew", which would suggest alternative arrows, one going in each direction between Matthew and Luke. I think this lay-out could prove quite confusing to students.