It wasn't a joke. It was merely an extension of an illogical train of thought.

Extruded food=food that swells, regardless of ingredients. As ingredients evidently don't matter, therefore it must be the extrusion process that makes the food swell.

Pipe is formed by the same process. Following the logic that ANY food+ extrusion=swelling food, then extrusion+ copper = swelling pipes.

Makes the "all extruded food is bad" argument seems a little nonsensical, dontcha think?