This behavior is fairly common among users because of the misconception that apps that appear in the multitasking screen are running in the background.
I know there is a multitasking screen on an iPad, however WhatsApp is not released for an iPad, it is intended for iPhone. So what do they mean by the multitasking screen on an iPhone?
It cannot be the list of running apps displayed by double clicking the home button because apps displayed there are in the background, though they may not necessarily be running. Is this however they are actually referring to?