As described, the movement consists of scapular retraction, which works the middle aspect of the trapezius muscle along with the rhomboids. Unlike the one-arm row, the lat is not involved. So as I said before, this exercise and one-arm DB rows are not interchangeable.
This is true if the scapula is maintained in a protracted (ie, rounded forward) position. If (as per the OP) the primary movement is with the back muscles, the rear delts will be only modestly involved (at best).
As an aside, I doubt if even the strongest IFBB pro could do true (ie, with scapulae protracted) rear-delt flies with 65# DBs.