There are no plans to close source anything. We're just going to build it privately, then open source it as soon as we launch

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that's good news!

But why not build it publicly, so other developers/volunteers can help?
Probably because trying to bring more people to develop new core features which likely requires a lot of quick back and forth and rapid refactoring would just slow everything down. Bringing more people to develop features on top of the core however is more desirable.