Neither C nor PyPy is "commonly used optimization strategy" so the comparison is very much correct.

People reach for C when they are desperate for performance, have the skills to do C, have the time to do C and are ok with loosing the cross-platform capabilities. Pure python code runs on mac/windows/unix (90% of the time). The moment you add a C module to the mix, you end up with the usual cross-platform mess (separate installers for each platform, complicated build processes etc.).