My off-the-cuff response is that, yes, of course the body is just as “contentful” as the blocks. But I also might suggest that that very point illustrates the unhelpfulness of the word “content.” It crams together two things that are importantly different. There are, I think, very important differences between “stuffing content into blocks” and assembling those blocks into bodies.

More broadly, I’d say that your intuition that authors are always assembling is right to a point. The authors I work with (mostly researchers writing longform reports and the like) are very good at assembling some very specific types of outputs, almost all of which are fundamentally derived from (and hence inherently bound by the limitations of) print. Indeed, the very example you suggest is fundamentally a print product.

Some of those forms of assembly are so deeply ingrained that many of my authors think that those just are part of the authoring process.

I think there’s a real difference between expressing an idea in words and conveying relationships between ideas. Stacking those ideas one after the other, separating them out with some paragraphs and some headings is one (very traditional) way of assembling. With physical media, it’s pretty much the only way. But it’s not the only way to do it once you bring a CMS into play.

That we mostly keep doing it that way is, I think, a consequence of asking authors to do the assembly.