I would suggest that the confusion rests on the the use of the term "Back plate". A back plate is usually the the plate on which brake shoes, wheel cylinders etc are mounted (series brakes typically) they are therefore an integral part of the brake system this would therefore be a valid reason for failure if insecure or corroded to excess . I believe you are referring to what is more properly known as dust or dirt covers fitted to disc brake systems which do not perform a braking function as such. I think the tester is mis-interpreting the test standards or regulations, some brake systems do not have these dust covers fitted from new. If the plates are insecure with imminent detachment likely then that may be a legitimate ground for failure, ask the tester to show you the section under which he is failing the vehicle!.
This is my opinion only.