Sure, and many people forget Helen Hindpere (second lead writer) and Aleksandr Rostov (art director) also left with Kurvitz. And even then, despite Rostov being responsible for the overall visual style the Thought Cabinet was painted by Anton Vill, and I don’t know for sure who made the portraits/skill depictions but it might also have been him (though they are more in the style of Rostov - see the 3 archetype paintings which were created by Rostov).
Look, I know it was a collaborative effort, but even disregarding the obvious centrality of these three people to the project the issue isn’t really about that. Kurvitz has been working on this world together with his friends since they were teenagers. It’s clearly his life’s work. Just because he was allegedly a shitty boss doesn’t mean he deserves to have the IP stolen from under him.