silly question: what's "world" about what's being generated here? is the an actual abstract representation of physical space (like, eg, a game-engine style scene graph?) or does it just mean "this video generator is more coherent physically than other video generators"
World in this context means that these videos are interactive, just like a video game. In the linked examples you can see the keyboard and mouse inputs. The model is trained to maintain about a minute of scene consistency so you can look around and objects out of view will reappear when you look back in that direction.
A world-model is one that predicts the next state of a simulated world given the current state and optionally some action from an agent inhabiting the world. It is quite analogous to a language-model that predicts the next word.
That world-state can be anything, but in the last year or two, the term has taken a narrower meaning: a video generation model that reacts naturally to game-like controls, as if it was simulating a videogame. But there's no additional state behind the video frames.