Usamah thought the declaration had been made merely to escape danger, but this was an inference rather than established knowledge of the man's heart. The Prophet ﷺ rejected taking a life on that basis, emphasizing restraint in matters involving human life.
تَعَوُّذًا here means seeking protection or refuge. Usamah's explanation expressed his interpretation of the person's motive, not reliable access to that person's intention.
Suspicion about why someone spoke at a particular moment and certainty about the person's inward belief are different. Decisions affecting life must therefore not rest on an unsupported assumption about what lies within the heart.