Not true. Just because they are Ghost type, doesn't mean they are actual ghosts of something that used to be alive. Gastly is pretty much just floating toxic gas that takes a ghostly form. I don't think it's ever stated that Haunter or Gengar are "real" ghosts anyways, more like shadows come to life.
Case in point, Banette. It is (supposedly) a child's toy that got thrown away that became possessed. For the most part, Ghost types could be considered species that are analogous to ghosts, but because they can breed and such, it wouldn't make sense for them to be ghosts in the sense of the spirit of a dead person.
Not to mention, wth would Rotom be a ghost of? And does your Snorunt die when you use a Shiny Stone on it?
The only Ghost Pokemon that has been canonically noted as an actual ghost (according to the Pokedex at least) is Yamask and the fact that it can breed implies some pretty ****ed up things.
Currently playing: Pokemon FireRed Nuzlocke [Charizard, Vileplume, Persian, Seadra, Hitmonlee, Raichu] Lvl 38, 5 comrades lost