He described his method on a forum post (Elitepvpers), and how it works is: He launches the original game (Metin2), which got an additional dll, and then he starts his self created UE4 game version of Metin2 with all the enhanced graphics and stuff you see in those videos. That dll is there to basically synchronize those two parallel running games, so that whatever he does in one of the games, will also happen in the other game (like, chars in the same level, doing the same animations, spells, actions etc.).
The only thing, where i was able to find something remotely close to rendering another apps in a game, are virtual desktops for VR games, where the desktop or a webbrowser or a specific app (like chat) is also displayed in that VR game. Not sure, if those are able to capture other games and send that data to UE.
Would it be possible to use a tv streaming stick or something similar (Miracast?) and use such a stream as an input for a media texture? Like instead of a webcam stream, whatever stream would be sent via miracast to any other app with a fitting receiver, just that the receiver is a virtual one in UE4 (if that does make any sense).