While Valve is finally bringing official Steam app to Windows Phone 8.1 and Windows 10 Mobile, it seems that the company looked to a third-party developer to help them get it done. Valve has apparently hired Thomas Cheyney, developer of the Unofficial Steam Authenticator to help it build its official app.
Cheyney, under the username Top_Cat, confirmed the move on Reddit. In answering questions from users, Cheyney indicated that the app, at least for now, would remain mobile-only.
This is certainly an interesting way for Valve to go about developing its official app. However, it's also nice to see a company pull from a talented community to get the job done. You can find the official Steam app now on the Microsoft Store, though you may not be able to download it just yet.