Istikhdām depends on a word being used with different senses in relation to a pronoun or another expression. Laff wa nashr depends instead on multiple items being matched with their corresponding attributes or judgments.
The earlier example that uses “sky” for rain while a pronoun points to vegetation illustrates istikhdām because two senses of an expression are operative. In the night-and-day verse, the two purposes are distributed across distinct referents without requiring the same word to acquire two senses.
The mere presence of a pronoun is not a safe test. Pronouns may appear in either environment; the defining semantic mechanism is different.