«تَرْجِيحُ أَحَدِ الْمَعْلُولَيْنِ عَلَى الْآخَرِ فِي إِطْلَاقِ لَفْظِهِ عَلَيْهِمَا»
Preferring the designation of one of two related things and applying it to both.
The point is linguistic usage: one expression is extended to a pair or mixed group without making all members literally identical in identity or properties.
To identify taghlib, determine (1) the distinct members or perspectives, (2) which form prevails, and (3) what the resulting form actually refers to. Merely noting a dual or plural is not enough.