it is factory option kit, but it was made by 'Altia', the REAL Nismo kit is actually quite different.
But if you were buying a R34 from a showroom, and ticked the boxes in the options list, you would be getting a Sports Kit, or Aero Kit. Altia is one of a handful of companies that get the details of new models early, so they can have kits ready by the time the cars hit the showroom. But you wouldn't have known the kit was made by Altia, it would have just been a Nissan option.