I read that as if to mean that it would be possible for the ISP to set a limit, say a 10 GB monthly cap, and once a lower threshold (say 5 GB) is reached, decrease bandwidth as usage continues to climb. As usage grows and bandwidth decreases, you could eventually reach a point where it is so slow (say at 1,000 bits per second) that it becomes more or less unusable for the consumer and therefore unfeasible to realistically exceed the 10 GB limit.
Sort of like halving the distance between you and a destination repeatedly ultimately leads you to a position infinitesimally separated from your destination, but never past it.