You can believe that if you feel like believing it. Doesnt make it true.The cassette provides housing or structure for the trigger. If the cassette is visible from the exterior of the complete assembled weapon it would meet the definition. The bolt carrier would also meet the definition. The upper receiver would also meet the definition. Maybe you will be right to assume they will only use this proposed definition to address 100% machined AR lower receivers but I believe they will take it much farther.