Jim Sterling's small disagreement with an asset flipping nobody game dev over displaying videos of games described the argument for youtuber uploads of gameplay. He said that the argument in court would be that the review or livestreamer playing the game is creating a transformative piece of work by playing it. There are an infinite number of different videos that could come from playing a game so any individual one could be considered transformative, combined with commentary and a face on top. That would fall under fair use. He went on to say the argument had not been tested in a court and he said it to demonstrate how expensive it would be for his opponent to try and control video reviews that were critical of the product via legal means.

I don't know the legal defense, a game developer is specifically trimming out bugs and separate paths through the game world so I personally would argue any experience a player has short of deliberately breaking the game and getting outside the map bounds should be considered the within the bounds of what a game is and therefore not transformative.