The roof strength test is performed "by pushing an angled metal plate down on one side of the roof" (https://www.iihs.org/ratings/about-our-tests#roof-strength-

The roof strength test is performed "by pushing an angled metal plate down on one side of the roof" (https://www.iihs.org/ratings/about-our-tests#roof-strength-test https://www.iihs.org/ratings/about-our-tests#roof-strength-t...). So it is the metal body structure that is tested here, not the glass in the center of the roof.