Not sure if that is accurate...
Most muscle cars have a horrible Power to weight ratio because they were so heavy. Most modern sports cars power to weight ratio far better than anything made during the golden era of muscle cars because of use of aluminum and carbon fibre construction in addition to turbo charging and fuel injection resulting in engines that were far more efficient and requiring smaller displacements to create the same amount of power.
Think it has more to do with body style and the fact that they were designed for performance.