exegetical puzzlement

Much of Paul Baumann’s review of Ross Douthat’s new book is devoted to intra-Catholic disputes that I won’t presume to adjudicate, but there’s one passage that strikes me as extremely odd:

Douthat is right about Jesus’ intransigence regarding marriage in Mark’s Gospel : “What God has joined together, let no one put asunder” (10:9). Matthew, however, finds this teaching too hard, and exercising his authority as an Apostle, an authority conferred by Jesus, grants an exception for “unchastity” (5:32; 19:9).

Now, the passages Baumann cites are from extended teachings by Jesus. So Baumann’s interpretation of those passages is that when Matthew claims to be reporting what Jesus said, what he’s actually doing is presenting his own teaching about marriage — but that’s okay because his authority to do so was conferred on him by Jesus. Which is one of the more peculiar bits of exegesis I have ever come across.