I did my own research on this issue, and I concluded that this passage is simply against to the Watchtower's soul-sleep doctrine. The book of Ecclesiasticus says:
And after this he [Samuel] slept, and he made known to the king [Saul], and shewed him the end of his life, and he lifted up his voice from the earth in prophecy to blot out the wickedness of the nation. (Ecclesiasticus 46:23)
This was the interpretation that ancient jews (like Josephus) held about that reading. They did not think that a demon was speaking, but the spirit of Samuel himself. Also, this is the natural reading.