Isn't this making HTML in some ways more like a PDF? Or am I missing the point?

Nope

PDF is an irreversible format in terms of editability. (btw I build the world's most performant pdf/pptx editor at https://eddocu.com , I would enjoy if you have any feedback)

Regardless, I cant find the relation in between.

It's like an abstraction that might help future genai models, or a packing strategy, or ..