Which is the entire point of the article: Most of the time even trivial video playback is done using JS due to the shortcomings of the <video> tag itself! This also means that browsers are unable to perform such optimizations.

> And just you don't need to limit all devices by the lowest common denominator either.

Unless the video site is intentionally sending cache control headers preventing caching of the video assets, this behavior is presumably already caused by the browser itself, not the site.