Official definitions of types of cars are bullshit. They're wrong and are either too specific or too generic, never exactly right. Muscle Car is defined as "any of a group of American-made 2-door sports coupes with powerful engines designed for high-performance driving."
That's incorrect because not only are there 4 door Muscle Cars, there are also Muscle Cars from all over the world.
If you do take it as correct, then a muscle car is also a sports car. Most cars are sports cars. You either have to be extremely precise, or accept that almost every car fits into almost every category.