He didn't need to mention it because it's implicit in data flow. Instead of the side effecting / state changing whip(cream) that needs to be under flow co

He didn't need to mention it because it's implicit in data flow. Instead of the side effecting / state changing whip(cream) that needs to be under flow control, you have whipped_cream = whip(cream) describing the flow of data. Data flow describes the relationship between non-changing entities and thus there are no side effects. While they could have mentioned it, it wouldn't really change the message.