You're totally right, but unfortunately, I think that's because the person writing these types of presentations don't have control over the content or design of the website, so removing stuff is often impossible or way more work that these tips.

The core holdback for slow websites is usually political, not technical or lack of skill. These presentations usually just try to make the best of those team dynamics issues.

It reminds me of "reduce, reuse, recycle, in that order". It's better to reduce resource consumption than to reduce its impact.