None of the Jaguars, including the XK's were known as muscle cars.
Muscle cars were a largely American phenom that peaked in the 60s, and were generally made to go fast in a straight line to take advantage of the wide open spaces here. Most of the money was spent on the drivetrain and the rest of the body and its options were fairly spartan to keep the overall cost low.
European sports cars relied more on finessed engineering and smaller sizes and weights to deliver their speed, and were generally faster around tight racing circuits.
Then there were the GT (Gran Turismo, or Grand Touring) cars, which were fast and luxurious, typically 2+2 two door coupes, and designed to travel great distances in comfort.
Over the years, improvements in engineering and more clever marketing have blurred the lines between these categories and now it is anybody's nomenclature.