When Miss Maudie compares Jem to Uncle Jack, she does so because Scout tells her that he has been telling wild stories about Boo Radley. We then go on to find out that Uncle Jack is known for egging people (especially Miss Maudie) on. This implies that if Jem merely repeats the story of Boo Radley being dead and shoved into the chimney to get a rise out of Scout, perhaps he really believes that Boo may be alive, and responsible for leaving the items in the knothole, but can't admit it to Scout, for fear that it might take away from his storytelling.
