No, this is not correct. The documentation states: 'indicates if an asynchronous continuation should be performed before the element is executed.'
If you try the async activity example in the distribution, you can modify the test case (ie remove the job execution through the mgt service) and you'll see that the activity never gets executed.
Shouldn't there be a warning somewhere if async is encountered but there is no job executor configured?
@Joram: Would make a nice and very interesting blog entry though, the difference between jBPM 3 and 4 regarding async stuff ;-)