I think someone is playing it, but it has a reduced set of inputs and they're playing it in a very specific way (slowly, avoiding looking back to places they've been) so as not to show off the flaws in the system.

The people surveyed in this study are not playing the game, they are watching extremely short video clips of the game being played and comparing them to equally short videos of the original Doom being played, to see if they can spot the difference.

I may be wrong with how it works, but I think this is just hallucinating in real time. It has no internal state per se, it knows what was on screen in the previous few frames and it knows what inputs the user is pressing, and so it generates the next frame. Like with video compression, it probably doesn't need to generate a full frame every time, just "differences".

As with all the previous AI game research, these are not games in any real sense. They fall apart when played beyond any meaningful length of time (seconds). Crucially, they are not playable by anyone other than the developers in very controlled settings. A defining attribute of any game is that it can be played.