It depends on the genre and if it's fiction or non-fiction. Non fiction, yes. But fiction is harder because of stylistic attributes and the fact it all has to connect. It's not like writing 300 300-word blog posts.
Too general a statement. Sure, an engaging consistent story is much harder than, say, the average cookbook or travel guide.
But many textbooks in science and math seem like absolutely Herculean effort. Gravitation[1] by Misner, Thorne, & Wheeler (a work of non-fiction) in my opinion would be the equivalent in intellectual effort to several thousand average works of fiction.
[1] https://www.amazon.com/Gravitation-Charles-W-Misner/dp/06911...
Ever read Dan Brown?
Is there? The general public can be sold any kind of crap that takes minimal effort -- and in the same way, don't buy masterpieces that take tremendous effort and skill to create.
>It might be tawdry, silly, and factually inaccurate, but those qualities are orthogonal to whether or not you can write a story that people want to buy.
This isn't an argument on actual writing skill though.
It's like you measure a new category like "skill of writing best-selling stuff" that's not necessarily connected to the skill of writing itself -- just to the degree of sales.
But one can sell bad stuff or skilless stuff too. Sales don't show anything more than a skill in sales themselves.