Yes, I agree with that. We used to have volume and flatrate tariffs for DSL. The flatrates used to be unlimited, then the volume tariffs were removed and the flatrates became volume tariffs with a high (but limited) volume.

Even according to English Wikipedia: https://en.wikipedia.org/wiki/Flat_rate

"A flat fee, also referred to as a flat rate or a linear rate, refers to a pricing structure that charges a single fixed fee for a service, regardless of usage."

Emphasis mine. I'd understand this as there being a service (e.g. high-speed DSL) which you pay a fixed fee for, independent of how much you use it. That does also mean that if you use it more they don't shut it off, because otherwise you're not getting the service independent of usage.