By now, Jem and Scout know that Boo "means no harm." But because of the specific night and the darkness, they need for him to be "gone." In the second paragraph of the chapter Jem says, "Bet nobody bothers them tonight" as he nods at the Radley house. Then Scout replies, "It is a scary place though, ain't it? Boo doesn't mean anybody any harm, but I'm right glad you're along."
From the very beginning of this chapter, the theme of superstition is evident. They begin talking of how dark it is because there is no moon. The cloud cover makes the strangely warm evening even darker. On top of that, it's Halloween and the children are walking alone to and from the school. All of these events are then supported by Cecil who jumps out to scare them on such a "spooky" night. So when Jem says that Boo must not be at home, it because that's what he wants to think. Harper Lee is setting up the reader for the upcoming climax of the story.
At the beginning of Chapter 28, Jem and Scout are walking towards the school on the dark Halloween night. As the two children pass Boo Radley's home, Jem comments, "Bet nobody bothers them tonight" as he nods towards Boo's home (Lee 157). At this point in the novel, the children realize that Boo is not malevolent. Scout then comments on the spooky nature of the home on the extremely quiet, dark night. Jem then mentions that Boo must not be home as they pass the Radley house. The reason Jem makes this comment is to illustrate how lonely and quiet the night is. Lee is setting the tone of the Halloween night and writes that the only two noises the children can hear are the sound of a mockingbird and blue jay as they pass Boo's home. Jem's statement is ironic because Boo Radley is at home and he will be bothered later on in the night when he sees Bob Ewell attack the children.