the link A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums isn’t for raw source code, it’s just for downloaded UE4 editor and futher packaging game right with editor, once you put steam api dlls into engine folder and configure you project for using steam online subsystem - editor will include steam dlls into packaged game without any extra actions
source code from GitHub usually for tasks when you have to add thirdparty code that isn’t already in UE4, there’s no need to download from GitHub if you want use steam, it already added to UE4 
just follow link steps to enable it