Not wrong, see the Windows 10 OneNote app for example, which already uses Typescript on WinRT/UWP. An Electron target will only be for legacy platforms, like Windows 7 and 8.
It's interesting they're using Electron for Win7/Win8 when React Native for Windows has a WPF rendered that works nicely on Win7 and 8. Perhaps they just found that maintaining two slightly different RN office apps for Windows wasn't worth the effort given the declining market share of the older Windows versions.