from what I've heard you're actually right. I'm not aware that somebody has specifically worked on this recently that would be able to share their work as it's low-level C++ code.
Cheers from the vrbn office,
I am not a developer but I do have an interest in getting CityEngine content in Unity (and other game engines). My intention is not to develop/replace SDK but to automate some workflows.
Workflow I am interested in testing and developing is an automation of processing assets from CityEngine (FBX is favourite for now) to create specific items like terrains, materials, collision meshes, prefabs, etc and then building a scene. That is first stage of automation. Then I would like to replace assets like trees, street lights, people, cars, etc with game optimized assets for optimization and visualization. Images below are from test scenes done as manual process to identify the issues.
For Unity editor scripting would be sufficient for those tasks. This something to get started and see how far we can get.