The guidelines are concerned with the average case: displaying the characters as *text*.
[The use of the word 'must' in a guideline is always awkward, since that word has such a strong meaning in the normative part of the standard.]
So, are you saying that for normal display of characters as text these guidelines must be followed?
No guidelines 'must' ever be followed. But if you want to follow the letter of the guidelines, you 'must' do this. ;-)
A better phrasing would have been: "These guidelines strongly recommend that you always do this, unless you have very good reasons not to, but in that case we hope that you are thinking of something else than ordinary display of characters as text, as you will sorely violate the expectations of your users and endanger the interoperability of Unicode encoded text from different sources."
Would that have been better?
A./
PS: perhaps we should have a little icon for that.
