i consider many post WWII hand brakes, specially on single circuit hydraulic brake cars, an emergency brake. I have used them for that purpose on more than one occasion when the hydraulic brakes failed, I did not loose control, rather I stopped quite smartly.
Showing an youtube video as evidence that a parking brake cant be used in an emergency without loss of control is akin to what you said of Miriam Webster.

Some parking brakes are too weak for much good in emergency braking, others work well. proper use and cable tensioning are critical. In the good ole days, some cars had no hydraulic brakes at all, only cable and lever operated, and for true sure stopping, everything had to be adjusted just right frequently to account for wear.