This is a semantic argument. No real winner either way. The term means different things to different people.
On one side you have the term "muscle car" painted with a wide brush to include any fast car. Fast car = Muscle Car.
On the other side you have a more precise break down where we can subdivide into "sports cars" such as Vettes, and "pony cars" such as Camaros, Mustangs, and Challengers, and "Muscle Cars" (Intermediates with large engines) such as Chevelle SS, GTO's, Chargers and GSX's.
You can pick a side if you want to but I don't think you can officialy "win" this argument.