"One of the goals for the Your Microsoft Account page was to share business logic between the web and native platforms."

Those aren't really the settings they're talking about. Whoever submitted this to HN messed up while editorializing the title.

They're talking about the settings associated with your "Microsoft account". Since it's a web "service", there is of course a web page from which you can manage that account. But they wanted to (also) let you do it from within the Windows control panel. That page of the control panel -- and only that page -- has been implemented in React Native.

Obviously this doesn't involve "rewriting" anything, since this is something that didn't previously exist.