I'm really curious how this was built. Quartz Composer got a lot of things right, but it's showing it's age. Was this rebuilt from the ground up?

Engineer on Origami here: Yeah, it's rewritten from scratch as a native mac app in ObjC++. Happy to answer any question you have :D

Pardon me if it's a silly question, but was Swift not a viable option for macOS development?

Swift wasn't ready when we started a couple years ago. We actually used some swift for internal things and it has been a bit of a pain, eg with the macOS SDK update for Xcode 8. Combining ObjC and C++ you get a lot of the benefits of swift like functional programming / static type checking etc, paying a similar price in compile time.

It would be hard to achieve the elegance of ComponentKit syntax (https://github.com/facebook/componentkit) in swift though. It lets us express the UI in a functional way :D