We don’t document each queue because there are a lot of them and we add to them regularly and sometimes remove some, etc. So it’s a lot of maintenance when it’s typically not that important for an operator of Sentry. You normally just run workers for all queues all the time. In special occasions you’d need to do more complex things.
But to answer your question, events.preprocess_event is the first stage of the event processing pipeline. So yes, what you say is correct. The server has accepted the event, and put it into the queue, but no work has been done on it yet.