Why does Jem say that Boo Radley must not be at home? What is ironic about this?

Jem's comment is based upon the fact that a bird is singing in the Radley yard, meaning Boo must not be there to frighten in away. This is ironic because before the night is over, Boo will save the children's lives.

Jem said, “Boo must not be at home. Listen.”
High above us in the darkness a solitary mocker poured out his repertoire in blissful unawareness of whose tree he sat in, plunging from the shrill kee, kee of the sunflower bird to the irascible qua-ack of a bluejay, to the sad lament of Poor Will, Poor Will, Poor Will.