Tested NW.js 0.15.0 with Steamworks SDK 1.36, the game works and shows up as being played on Steam, but no Steam overlay. I read the thread and apparently there's still no fix for newer NW.js versions? Adding --in-process-gpu did not work unfortunately.
Edit: I got it working. It won't work if you only export, modify and run the game in any folder. You have to run it through Steam. I just went to my game's folder on Steam, deleted everything and copied the new files over and it now works.