I believe Bevy (rust game engine) does leverage multi-threading for running game code (most 'data oriented' frameworks do). The most popular older engines like Unity do not (unless using the experimental DOTS framework).
So, the phrase "a tremendous amount of game code doesn't benefit from multithreading" is actually true since most game code is in unreal/unity, but it's not a hard constraint for all game engines.
So, the phrase "a tremendous amount of game code doesn't benefit from multithreading" is actually true since most game code is in unreal/unity, but it's not a hard constraint for all game engines.