Well, ALL "domestic" duck breeds except for Muscovies and possibly the Black Cayuga are descended from mallards, but I'd say basically, this person just doesn't know the terminology well.
To me, if you said "domestic mallard" I would take that to mean captive raised and bred wild strain mallards that would mature at 3-4 pounds and be fully able to fly - genetically the same as the pure wild birds found in nature, just raised in captivity.
Versus a Rouen which is the product of intentional selective breeding by man for size and meat quality, weighs between 6 and 12 pounds, depending upon the strain, and is essentially flightless or can take only very short flights due to its body mass.
