It's not Electron, it's using Edge WebView2 which is drastically different from MSHTML.dll in many ways. If you have it installed you can see the msedgewebview2.exe processes in Task Manager.
Agreed that it appears WebView2-based (at least on Windows). I apparently have the new Outlook already on my computer (how? Not sure!) and I see WebView2Loader.dll but not LICENSES.chromium.html.
Huh - okay, I retract my claims about it using Electron.
...which I based entirely off the fact the right-click menu used Chromium's non-native-menu-styling - along with MS' history of using Electron elsewhere.