Blender Game Engine APi

So, I found out that two or more games have actually been published on Steam. I was wondering how that happened? I read that they used a custom APi for their publication? Can anyone explain how or what they did? I have looked everywhere for an answer but cant seem to find one.

If you are going to tell me that BGE is obsolete and I shouldn’t even use it… Please don’t bother to reply.

You can publish on steam without using the steam overlay (e.g. Age of empires 3). As long as the license is compatible, you can deploy on steam.

Does that mean use the Runtime feature already in the program? Can you publish to steam with out any additional add-ons?