That’s not correct. They immediately detect a queue item being added and immediately start the job.
The 30 minute polling is when you have things still sitting in the queue (because your automation faulted). For example, you have a queue trigger to start one job when one item is added, and you add 10 items. It starts the job immediately. Part way through processing, your job faults and leaves 5 items still new in the queue. This is when the polling is relevant. Next time it polls it’ll see that there are still new items in the queue and start a job.