I agree, and one thing that makes me skeptical is that for it to be effective, the fish being punched would presumably have to understand that the octopus wants

I agree, and one thing that makes me skeptical is that for it to be effective, the fish being punched would presumably have to understand that the octopus wants it to act in a certain way - i.e., the hypothesis seems to imply that the fish has a theory of mind with respect to the octopus. Similarly, it seems to imply that the octopus has a theory of mind about the fish - it believes that the fish will understand why the octopus is punching it. I'm not saying that this is impossible, but it is a very big claim. Furthermore, it is not clear from the video that the punching has the outcome implied by this hypothesis. It seems at least as plausible, at least from the video alone, that the octopus is just keeping the fish away from stealing an anticipated meal.