Is it true or something different material?
I've confirmed that it was made of "plastic" but not what type yet.
The combustion chamber assembly consists of an engine case and mount assembly and a plastic assembly, which includes the nozzle extension. The engine case and mount assembly is bonded and locked to the plastic assembly to form an integral unit. The plastic assembly provides ablative cooling for the combustion chamber; it consists of the chamber ablative material, the chamber insulator, the nozzle extension ablative material, and a structural filament winding. The chamber ablative material extends from the injector to an expansion ratio of 4.6. The chamber insulator , between the ablative material and the case, maintains the chamber skin temperature within design requirements. The ablative material of the nozzle extension extends from the expansion ratio of 4.6 to 45.6 ( exit plane) and provides ablative cooling in this region. The structural filament winding provides the structural support for the plastic assembly and ties the chamber and nozzle extension sections together .
Source "LM10 Handbook Volume 1" page 2.3-41 and 2.3-42 (pp. 296-7 in the 800+ page pdf)
This schematic confirms that the "structural filament winding" mentioned above was fiberglass.
Source: Apollo Experience Report: Ascent Propulsion System