I think it’s a reasonable message - we should be teaching our dcs that if anyone wants to get access to their privates it has to be for a valid reason, and saying “I’m just going to change your nappy ok?” is probably something we all do anyway.
Once the child is old enough to say no, o guess it brings up a difficult situation regarding pressing on anyway, but no more so than a child needing an injection and you/the nurse explaining that it has to be done and that you’re sorry.
It’s a bit like the advice to stop tickling etc as soon as a child says stop. It’s just bringing them up with good boundaries and sense of bodily autonomy. Nobody seriously expects a newborn to answer back, but it sets a good precedent for people asking persmission to touch their private areas.