The commentary also discusses the possibility that an indefinite sayyi’ah suggests smallness or rarity. In combination with in, this presents a particular misfortune as nonroutine in the intended comparison.
Indefiniteness does not always mean scarcity, nor does definiteness automatically mean abundance. The possibility here is inferred from the juxtaposition of terms, the verbal context, and the way the condition is framed.
Conditional particle selection and nominal definiteness are separate rhetorical devices that may reinforce one another.