From their GitHub:
cpak installs applications from OCI images while keeping package metadata in a Git repository. It provides native desktop integration, shared content-addressed layers, atomic updates and a rootless Linux sandbox from one Go binary.
It seems interesting. I’ve never heard of it before, but apparently Bottles packages with cpak along with flatpak. The main reason I use flatpaks are its sandboxing capabilities, so it would be cool if cpak brings new stuff to the table while matching or overpassing the sandboxing capabilities.
The way I read that is they don’t want forks fixing specific issues/making improvements when those changes could all be in the one implementation.