There's 2 classpaths here. One is the classpath used by the ant taskdef and the other is the one used by the execution of the ant task. The ant taskdef's classpath is the one that will locate the hibernate ant task tool.
For more info on taskdef classpath check the ant taskdef docs. It's pretty straightforward.
Bjoke: A "Bully Joke". A Statement or action made with malicious intent - unless challenged. At which point it magically transforms into "I was just funnin'" or "What's the matter, can't take a joke?"