I was under the impression that ZA/UM owns the rights for not just the game but the world of Elysium, though I am not a copyright lawyer or anything so I don’t know what exactly is copywritable.
There were six writers, and dozens of other people, for most of the development of Disco Elysium. Why would it be any better for just three of them to get the rights?
As far as I know the world of Elysium was created and developed as a setting for a D&D game Kurvitz was DMing for his friend group starting from his teens. That group of people built it together over two decades. Kurvitz also wrote a novel set in that world, which released in 2013. The setting already existed and was pretty developed when work started on Disco Elysium the video game. If you want to get into the weeds I agree that people like Hindpere and Rostov have less of a claim on the IP than people like Martin Luiga and Argo Tuulik who were part of that campaign DMed by Kurvitz. Tuulik was the last writer who worked on DE remaining at ZA/UM, but he seems to have been fired the other day. The interview with him was a good read.