Interesting article. It is clearly possibe to speak with the dead as the bible tells us in the Old Testament Book 1 of Samuel verses 1-28 where the woman of Endor brought up Saul, and the bible tells us that this was not a demon but indeed Samuel. “And Saul understood that it was Samuel...”
The issue is that we aren’t supposed to - not that it can’t be done. Necromancy forbids the conjuring up of spirits in order to tell the future and this is indeed a serious sin if one were to do it. However, the fact remains that the bible would not have bothered to forbid the practice if it were impossible.
“woman of Endor brought up Saul” was supposed to say:
woman of Endor brought up Samuel