the problem with quoting White Dwarf is that its articles are often written before final versions of rules are in play. Or the authors may have gotten something wrong by accident.
An example would be the apocalypse battle report where the tau super heavies managed to repair themselves, AND fire in the same turn. This led to one munchkin trying to do the same with his baneblades becasuse 'it happened in the white dwarf and that overrides the rules in apocalypse'.
So in the WD article the DK is said to carry both two Doomfists, both of which have storm bolters. However in the codex those same bolters are not listed (but are on the dreadnought). furthermore, if the DCCW works as it does on the dreadnought, then why do you need the daemon hammer upgrade?
As such I'm of the mind that the doomfists are just there to give it an extra attack should you choose to try and keep it cheap or equip it as a gun platform. Pretty sure DCCW are like powerfists in regards to getting extra attacks for multiple CCW weapons.