So if I'm reading this correctly, the author is using a queue system (Celery, presumably with a backend such as RabbitMQ?) to run background jobs, but then wait

So if I'm reading this correctly, the author is using a queue system (Celery, presumably with a backend such as RabbitMQ?) to run background jobs, but then waiting for them all to complete before finishing a task. In effect, you sorta get concurrency (assuming multiple workers are processing jobs). Or does Celery also provide a way to do threading without using a backend service (e.g RabbitMQ, beanstalkd, sqs)? Basically, I'm confused.