Although malt whisky can be made using other malted grains besides barley, those types are not called malt whisky without specifying the grain, such as rye malt whisky or buckwheat malt whisky.
The statement emphasizes that malt whisky is a specific type of whiskey made from malted barley.  It clarifies that while other grains can be used to make malt whisky, they must be specified when referring to it.