Of course anyone could have run regular old Dolphin through steam by adding it as an external app. People have been doing that for years now.

This is a full blown steam release. And will have all the bells and whistles that come with running an app natively through steam that don't work with the old method. Retroarch did the same thing and launched a Steam version few years back. More options are better but for the most part, the standalone desktop version will still be the definite way to play.