saving come from
1. code reuse. less/stylus will generate everything dozens of times
2. vendor prefixes are added effectively at runtime
Preprocessors will generate bloated CSS if not coded properly, same as it's possible to hand-code bloated CSS.
There are libraries that generate vendor prefixes at run time. If prefixes are really a bother (they're not as much anymore) then preprocessors and other compiling tools have solutions for that as well. Which will be more efficient for the browser.
2. I know code reuse technics in css, they are mentioned in the presentation. Cascades do not work for component based development. Adding multiple classes to an element leads to awkward code and also will lead to namespace conflicts if you don't name everything very specifically.
3. Vendor prefixes are there and will be used for new features too. They will not go away. Precompilers can't avoid generating all prefixed variants.