لِكُلِّ مَقَامٍ مَقَالٌ.
Every situation calls for an appropriate expression.
The short sentence communicates a broad principle: rhetorical merit requires fitting words to circumstance. There is no need to invent a missing syntactic element to explain its economy.
Ījāz al-qaṣr achieves breadth of meaning through concise wording without an assumed ellipsis. Brevity must still preserve a clear, complete intended sense.