Are you sure this is even correct? The github project for that 3d demo (https://github.com/kripken/BananaBread/) says the 3d engine is "Cube 2/Sauerbraten 3D", not Unreal.
It seems like the Unreal Engine facebook just posted a link to that video and people are thinking it is Unreal Engine.
BananaBread is a different 3d engine, it's what most of the development work of emscripten has been focused on. The UnrealEngine demo is a separate demo done by compiling the UnrealEngine codebase to asm.js using emscripten.
It seems like the Unreal Engine facebook just posted a link to that video and people are thinking it is Unreal Engine.