The prohibition of riba in wa-harrama al-riba is clear. But the brief wording alone does not spell out every legal form of riba in commercial exchange. The marginal explanation says that the Sunnah further clarified this subject, so the example is placed under initially indeterminate specification.
“Indeterminate” does not mean uncertainty over whether riba is prohibited. It refers to working out which transactions fall under the prohibition, a task requiring additional legal clarification.