I don't think they were appreciating that HTML could be read unrendered. I think they meant that it was up to the browser to render HTML with sensible but unspecified or otherwise user-specified styling (the browser is supposed to be a "user agent", remember?) before web designers started aiming for pixel-perfect control through CSS.

That is an unjustified over-generalisation.

HTML markup is pretty readable (except when it becomes soup) and I read and write raw HTML documents all the time. I like it better than markdown.

It's even more readable in a code editor that distiguishes tags from content.

On this planet, humans have read HTML without parsing for years. People building their first websites without any significant technical knowledge stole HTML by reading the source of other sites and edited it by hand.