The core is obviously not an allowed material. The Plank Specification along with its Interpretations is one of the most clearly written specs in the class.
There are many specs in the class that rely on the integrity of the builder since they can’t be inspected or measured. This does not take away from the requirement to meet the specifications.
In my opinion a builder that has so little regard for the specs should not be allowed to supply to the class members.
We had a builder that was involved in a collision at a regatta, and it was found that his fuselage side panels were constructed with an inner layer of Kevlar. If I recall correctly that builder was disqualified from the regatta and removed from the IDNIYRA.