Riba is linguistically connected with increase or excess, but not every increase in a transaction is riba. Sale transactions may involve prices, gain and profit; the mere idea of an “increase” does not place all of them in a single prohibited category.
The textbook's footnote explains that unrestricted increase is not the legal sense intended here. Rather, a particular kind of increase is meant, to be determined by authoritative clarification. The note points to the hadith concerning six commodities as part of that clarification.
The usuli lesson is that an everyday meaning may not provide the boundaries of a technical legal expression; those boundaries must be established through evidence.