See my full narrative in the original thread (this one is just a shameless repost): https://news.ycombinator.com/item?id=17728515
They've revealed that they're using RN for all their main products now, but never explained how they were targeting macOS. This is the repo they must have been using; and surprisingly, it's unrelated to the repo of the same name that has 14k+ GitHub stars.
What's brilliant too is that they are indeed open-sourcing it.