The commentary explains that bi-lafz fasih is not restricted to a single word in this definition. The wording can be a simple expression or a composite utterance.
A single expression: naming something with an appropriate fasih word.
A composite expression: stating an event, relation, or judgment in a properly formed Arabic sentence.
Speaker-fasaha should therefore not be measured merely by the length of a speech. The relevant question is whether the intended meaning can be rendered in fluent and correct wording of the required kind.