The functionality in Computer Vision System Toolbox is available as MATLAB functions, System Objects, and Simulink blocks.
It's implemented in a mixture of MATLAB code and C code, and as far as I know does not rely on OpenCV at all.
However, it also supports Code Generation using MATLAB Coder and Simulink Coder - in other words, once you have prototyped and developed a system using MATLAB, Computer Vision System Toolbox and maybe Simulink, you can automatically generate much faster C code from it (which, if you want, could be integrated with functionality from OpenCV).