Actually it looks like what may have happened is Microsoft is using React Native in their Office apps (to some extent) and with this project are trying to open source it.
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.