Note that the report says "derived from witness marks on the preload plates", so the investigators did not use witness marks on propeller blades to determine the blade angle. This might have been possible, as scratches and the deformation of the blade tip do, to some extent, manifest blade angle, but you would need to take into account the speed at which the plane was travelling at the moment of the propeller strike. There is also the problem that propellers tend to get quite mangled in accidents such as this, so the endresult may be useless as evidence.
Now, the preload plate is more suitable for determining the angle of blade at the time of impact. It is a part at the root of the propeller, inside the propeller hub:
As the propeller hits the ground (or any solid(ish) object) the force of the impact is relayed all the way to the bottom of the blade, where the preload plate sits. This force will leave an indentation, a witness mark on the preload plate, and from this mark the investigators can determine the angle of the blade at the time of the impact, as the place of the mark will be specific to the blade angle.
Since it was briefly discussed in comments: A witness mark can be both a deliberate marking on a part, made to ensure proper installation for example, but it also means a mark induced by force during an accident or some other event:
Waywordradio.org: witness mark (thanks Chris)
In good old days one common witness mark the accident investigators were searching for, were the small scratches the gauge needle made on the dial during an impact. These witness marks pretty accurately showed what each gauge was reading at the moment the plane crashed if the impact was at a favourable angle.
Picture source: Propeller Owner's Manual - Harzell Propeller Inc.