Personally, and not factoring in the fact that we only learnt what ghosting actually entailed many years after HttE, I think he was spinning Luke a line. It is Obi-Wan, after all. But I'm also content to think that the ghosted Jedi isn't perfectly in step with the will of the Force, but rather in step enough that the process of dissolution is incredibly slowed in comparison to the typical person or Jedi. Either way, as Qui-Gon says in the RotS novel, it can be achieved only through selflessness, not greed, and involves retaining one's influence while actually being one with the Force. Not holding off becoming one with the Force.