«أخوك» in «زَيْدٌ أَخُوكَ» is definite through annexation. One approach explains the definiteness with reference to an already identified relation or individual; another examines the kind or relational meaning supplied by the expression.
The issue is not merely whether annexation is grammatically present, but what kind of definiteness is involved. Distinguish conceptual familiarity from external identification; otherwise separate analyses can be mistakenly forced into a single interpretation.
If the hearer already knows the asserted identity, neither reading automatically supplies new information. The usefulness of a report remains tied to a genuine informational gap.