10.2.3. The Event interface - provided executor cannot execute observers notification

If the provided executor cannot execute observers notification, an IllegalArgumentException is thrown.

What does it mean "cannot execute"? I am not really sure. Does it mean the observer throws an exception which should be wrapped in IAE?
This would likely contradict 10.5.1. Handling multiple exceptions thrown during an asynchronous event.
Maybe this sentence could be omitted.