That statement is very incomplete, as it entirely ignores the role of software. As things are now, you will see no performance difference between the dual core S4 and the quad core S4 (except in theoretical benchmarking apps designed to stress test multithreaded performance). That isn't changing anytime soon.
The best shot at exploiting any performance gains to be had from a quad core CPU's would necessitate Microsoft specifying a new chassis spec for WP9 mandating a quad core CPU. As long as dual core CPUs are part of the WP8 market, developers won't be taking steps to exploit quad core CPUs to any significant affect.