Rosenstock-Huessy says, quote: "Mark states bluntly that he is quoting from Matthew". That's it, no explanation as to where Mark states this, but I have a few ideas which need more thought.
If he “states bluntly”, it means it will be pretty obvious where the statement is. Mark doesn't state anywhere that he is quoting Matthew (at least not the gospel we now call Matthew). Your source is lying.
Matthew introduces Jesus as 'Christ, the son of David, the son of Abraham'. He concludes by saying (28:19) that Jesus is the Son of God (the second person of the Trinity). This is the point to which Matthew's gospel has progressed: the last step of thought. Mark begins by introducing Jesus as Christ the Son of God, thus beginning to think and to speak where Matthew had ended, and turning Matthew's last 'word' into an opening of a new drama.
The attempt to make a developing Christology from the synoptics in this way is very flawed, though. Mark may open his gospel indicating that he believes Jesus to be "The Son of God" (and maybe this opening verse's authenticity has been questioned, along with the 'Long Ending') - but in his account of Peter's affirmation of Jesus' true identity, he simply uses the words 'The Christ'. That would seem to be a back-tracking on Matthew, who adds "the Son of the living God". And Luke has only the words "The Christ" also.
No doubt, by the time the synoptic authors wrote, they had all come to consider Christ to be a divine personage in some sense, but the implication is much weaker in Mark. And Matthew's addition of theatrical effects rather obviously suggests that he is tarting up the plainer narrative of Mark to heighten the drama - to the extent that during the crucifixion story, his chronology is all over the place.