«فَكُلُّ مُدَبَّجٍ أَقْرَانٌ، وَلَيْسَ كُلُّ أَقْرَانٍ مُدَبَّجًا»
“Every mudabbaj case is peer narration, but not every case of peer narration is mudabbaj.”
This is a distinction between a broader and a narrower category. One-way narration among peers establishes aqrān; verified two-way narration adds the condition for mudabbaj.
Case 1: Zayd narrates from ʿAmr and they are peers. This is aqrān; mudabbaj remains unproven.
Case 2: Both Zayd and ʿAmr narrate from one another. Both descriptions apply.
Calling every peer relationship mudabbaj overlooks the extra evidence demanded by the latter term.