It's neither Windows 10 Mobile nor Windows RT. Daniel Rubino has explictly covered these issues before.
Windows 10 S is identical to any standard Windows 10 version, except with one switch flipped, to stop you or someone else from screwing up your system. In fact, this switch already existed way before W10S was announced, and can be found in every Windows 10 Settings. Essentially, you already had this very experience, but in reverse: You had Windows 10 and were able to make it Windows 10S with one switch.