All his men - This and similar expressions must not be taken too literally (compare 1 Chronicles 10:6). We know that Abner, and Ish-bosheth, and manymore survived the day of Gilboa.
Related Commentaries of 1 Samuel 31:6
1 Samuel 31:6
6 So Saul died, and his three sons, and his armourbearer, and all his men, that same day together.