This doesn't proscribe or prescribe "thou shalt not/must always", it is an example thay says "Shit's hard, yo. Don't expect easy wins."
Even my "solution" (separate instructions and data by having an LLM write a program to process data, never touch data directly) is at best going to be like a philosopher writing a dentological ethics book that gets implemented by extremely literal-minded jobsworths.
In any case I don't think that's what they're saying, because they presented a false dichotomy in the original example.
> Would you want the human assistant to just dismiss this as a prompt injection attempt? Or ignore it because they were told to treat e-mails as data and never act on them?
There is a third option, have the assistant raise to the person they are tasked with assisting.
Like all emergencies, it's a low probability event with extremely high impact. You don't want people to ignore them, in fact people are trained - by their public services and their employers - to not ignore them and how to react efficiently.