Problem 4-83

b. HCN is a "weak acid"

This expression is used when an acid is partially dissociated in water.
In the case of HCN, its %dissociation is approximately 0.002%
Therefore, since 99.998% of HCN is not yet dissociated but soluble, the solution manual writes HCN(aq)

You are not supposed to know this yet. (acid-base + equilibrium chapter 13 and 14 covered in NYB).
Therefore, writing the total ionic equation:

H⁺(aq) + CN⁻(aq) + Na⁺(aq) + OH⁻(aq) → H₂O(ℓ) + Na⁺(aq) + CN⁻(aq)

at the moment, in NYA, is perfectly acceptable.