I agree, the two labels I gave aren't that great in retrospect, sorry :)
whether it comes from the same hand or is a later addition is debatable, but even if it does there is quite a leap from it to the Pharisaic and Christian idea of post mortem individual hope.
The reason for this leap can be debated, but for JWs who believe that all things are revealed in Gods word seem to take the opinion that Old Testement writings are on the same level as things that God would reveal later, or so it would be in their minds. It's almost as if they go back to the 'old light' or partial understanding that came before the better explanation.
By the way, isn't their an old Jewish commentary that explains this scripture as meaning spiritual death? I can't remember where I came across that.