Its very rare to not have a pure Muscovy. However there are some lines coming out of the Orient. Im sure to get the very large whites the French did some mixing as well. The line I have is as pure as any in the US. I don't know what that says as many back yard flocks to let ducks run together and if they cross they cross, but on average, they do not.
I have wondered myself if the ones with the very dark blood red under body weren't a cross with Rouen, or Mallard, however they are very large, and would not look and produce like Muscovy if this was the case. Like you, at one time, I did question it.
I have also had it suggested that the hen was a silver. The person had only seen the duck in photos. In real life, I assure you she is white. She was just like the other white ducklings when hatched. This peach color only shows it's self in the same lines as the blood breasted birds. A mutation I have yet to have anyone identify.