[tromp]

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.