The judgement about whether an email needs a response is left to the receiver who would be responding, in their statement. I like that mental model of things. :)
That leaves room for miscommunication, though. If someone expects a response from me, I'd rather they make sure to ask it at the very top so that I don't miss that request with everything else that's going on.
Re-reading the statement, maybe they simply mean replies should be above the original e-mail rather than inline?
>28. If an email needs a response, write the answer at the top of it.
Alternatively, it feels like it could read:
> If an email includes a response, place it at the top of it. If an e-mail requires a response, also make that clear at the very top.