Earlier quoted context omitted.
When done properly, it increases the separation between the actual content, and the styling on top of it. Any time you get closer to that ideal separation, accessibility is enhanced (and it’s also forward-thinking towards yet-unknown methods of accessibility assistance).
IMO this is an example of extreme overengineering and premature optimization. Unstyled html is so ugly and inconsistent across browsers that it’s just not a realistic scenario that it would be consumed without the css and js that make up the rest of the code. When an alternate display method comes out and wants my semantic html, I will add it to my QA workflow and make sure that it looks good instead of relying on so…
Sure it's mostly black text on white backgrounds with only minor typesetting differences between elements but as a means to present information it has worked since the mid 1400s...