What objection is made to deriving a universal rule from “al-wāʾidah wa-l-mawʾūdah fī l-nār”?

Salvation of Children: Aṣlaḥ, Recompense and Theological Differences

• The terms or context of the narration may refer to a particular case
• The word “Fire” must always mean a worldly fire
• The wording itself determines every child’s precise age
• Parental affiliation alone establishes the report’s authenticity