To the best of my understanding, it is exactly what I wrote in the parent comment. It's the fact that the core lacks bits developed in the internal tool but not yet unglued from what will go in the plugins section.

It's not unheard of. For instance, I got another team and devtool I work with, non-related to the API niche, and they are going open source mid-next week. Some teams want to build the core their way, some want to see if it will catch traction before committing to support an entire community and contributions, some have got something else going on, some actually ARE shady and talk OSS for clout, etc.