What is a book? I mean, I spent some time over the last couple of days reading parts of the PostgreSQL manual — if you printed it it would be a gigantic book but it's online and linked. Does that make it not a book? Half a book? Still a book?
A blog post can be a few hundred words or a hundred thousand. It can even be a series of blog posts. Is that now a book?
David Foster Wallace talks about reading as being an activity that requires you to sit quietly alone in a room and engage with a piece of writing. I think the act of doing this is quite different to reading the postgres manual, despite both of these tasks requiring ingestion of text.
I do think the publishing medium is relevant and not just word count. Books tend to be better researched, edited, triple checked and well planned out to convey the full information they want to. This isn't always the case but there's an inherent push in this direction when you're publishing a book under your name that you want it to be good.
On the other end of the spectrum we have social media posts which are anything but.