I think people are reaching to form a narrative behind the possibility that Office is gonna become an awful unusuable Electron app.
Not Electron involved at all. The point of RN is that it is native; no overhead of running headless Chromium. The fact that Microsoft are putting their weight behind it means great things for RN, desktop, and macOS. They say they've finished rewriting most of their core apps with RN; this is the first indication of the codebase they likely used to target macOS. It's come out of nowhere, too: there's a same-named repo with 14k+ stars, for comparison.
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.
So, an Electron app...