Not calling you a lier but I've never seen that to date. Maybe your car had the old type pads that required those shims and the installer failed to install them?
The cars we see with obvious rear brake resonance have warped (as measured by a run-out micrometer) or very rusty rear rotors that require replacement with new pads and those cars then stay fixed, do not return with the same problem.
Being new OEM parts with a 1 year, unlimited mileage parts warranty, we'd see those customers return if they still had any rear brake problems within that time period and that has NEVER happened once at our dealership so far and I've worked in Volvo dealerships for over 18 yrs now.