Interesting. Any docs that explain what/how they do this?

A combination of LISTEN/NOTIFY for instantaneous reactivity, letting you get away with just periodic polling, and FOR UPDATE...SKIP LOCKED making it efficient and safe for parallel workers to grab tasks without co-ordination. It's actually covered in the article near the bottom there.

Good Job is a strong attempt. I believe it's based around Advisory Locks though.

https://github.com/bensheldon/good_job