It is a fan design.
The Phobos Pattern, a semi autonomous exo-suit, was designed for normal human proportions, and for deployment amongst the Imperial Guard. The suit was not intended to replace or even come close to an Astartes armour, but act as a 'force multiplier' to bolster IG ranks and to allow once disabled soldiers to fight as equals alongside their own. Once complete, the Phobos MK1 plans were presented to the Imperium, and deemed absolutely heretical. As a result, his Forgeworld was to be swept by Space Marine kill teams in a purge mission. The Older brother sent to kill the younger brother as ordered by their Father.
The small Astartes force that was sent to wipe them out realised the visionary aspect of the project during the mission briefing itself, but continued their mission without question for fear of being labelled traitors.
Eventually, the Astartes force was reduced to the last Marine, a Sargeant who called down an orbital nuclear bombardment when the mission was deemed a failure.
In his final moments he looks on as a shuttle escapes the facility and into the warp, and with it, the construction template for the Phobos Pattern armour. That was his only act of defiance to the Emperor in his lifetime as a Space Marine, one that he hoped would help pave the way to a brighter future for the Imperium.
The shuttle emerged from the warp years later, and found itself in Tau space. The Tau, learning of its origins and potential, openly embraced the Phobos Pattern prototype and began additional R&D work to implement it into their Gue'vesa/Human Auxillary force.