With all due respect to the authors of that article I'm sticking to the
Three Amigos in all matters UML. The "Unified Modeling Language User Guide", Chapter 17: Use Case Diagrams
Use case diagrams are one of the five diagrams in the UML for modeling the dynamic aspects of systems (activity diagrams, statechart diagrams, sequence diagrams, and collaboration diagrams are the four other kinds of diagrams in the UML for modeling dynamic aspects of systems).
Their definition of "dynamic" may be simply "not static". Chapter 16: Use Cases
A use case describes a set of sequences, in which each sequence represents the interaction of things outside the system (its actors) with the system itself (and its key abstractions).
A sequence is neither instantaneous, nor permanent. So it takes place over time - not necessarily properly quantified, nor the order of the steps identified.
[ November 11, 2005: Message edited by: Peer Reynders ]