Without having watched all the video, it really depends on where you are going.
Companies such as Amazon and Google profits relies on the speed they deliver a page. Of course CSS is just one thing and much more optimizations can be done. But for the maintanability knowledge sharing, semantics should also really be kept as much as possible or at least the company should have explicit documents explaining their view on semantics and when it differs and probably why - engineers question a lot, that's the nature.
Yet other companies such as public institutions are not so depending on speed but often it's often crucial for way backwards compability. Here semantics should be weighted over speed. Not saying Google shouldn't consider older browsers, but if their audience has a 0.01% of IE 6 users or less, they might try campaigns to convert these to a more modern browser instead. If they can see a profit by doing so of course.
The solution, or compromise if you want, could be tools such as Webpack. Here you can develop in a completely perfect semantic order, but when Webpack is run it can re-organize the structure to be more speed optimized. That's already what it does when you minify.
The magic word here could be: Compiling.