Resolution:
Event.assertEvent() asserts the event into BE Working Memory immediately without going through any destination.

Event.sendEvent() sends the event out through default destination. The event can be in BE Working Memory through the event destination if the destination is enabled as input destination when configuring the BE Archive.

A newly created event won't be in Working Memory unless you assert or send it.

A good example you want to use assertEvent() is when you only send the event to other rules with in the same rule session. If you want to send the event to somewhere outside the rule session, you need to use Event.sendEvent().