In benign environments, the environment might be random. It might be stochastic, but it has no objective on its own that would contradict the own objective. So, for example, weather is benign. It might be random. It might affect the outcome of your actions. But it isn’t really out there to get you.
Contrast this with adversarial environments, such as many games, like chess, where your opponent is out there to get you. It turns out it’s much harder to find good actions in adversarial environments where the opponent actively observes you and counteracts what you’re trying to achieve relative to benign environment, where the environment might merely be stochastic but isn’t really interested in making your life worse.