I wish they actually wrote about their exact implementation. Article is kinda light on any content without that. I suspect you are right, I have implemented this kinda thing in a similar way.

what happens if the task cannot be completed? or a worker goes down? Is there a retry mechanism? maintaining a retry mechanism might be a huge hassle.

It's a bit of work but allows for fine grained retry logic. Some jobs should not be retried but escalated, others might need a retry in one hour, others can be retried at a lower priority. I prefer a flatter stack and total control over the queue. That being said, I have RabbitMQ running in a production project that I haven't touched in years, it's rock solid and I'm pretty sure these guys had it misconfigured.

For something this size, my guess is it creates an alert and somebody looks at the problem.