A bit of reading comprehension will show "20-shot clip magazine"
clearly stated.

Feel free to take it up with folks like Thompson, Remington &
Browning though.

OTOH Is that the type of magazine Gummer needs in his outhouse?
Why?

That article is either in error or uses some kind of local or archaic
terminology. Instead of clip magazine they mean box or straight magazine.

There are "clips" in use with guns, but that are open metal strips. Usually
used for holding rounds while loading rifle magazines. I believe some very
early stationary machine guns used open clips that slid into a kind of
horizontal tray, however that's more the exception than the rule.

GTO(John)