The edge of LW over a specialist textbook is, in my experience, about the audience that content is written for. All writing is shaped to some extent by the profile of the expected reader. The expected reader on LW is likely to be bright, curious, and epistemically exacting, while not being a specialist nor intending to specialize in the field of the any particular piece of writing.
Intermediate and advanced textbooks get to assume that the reader has already invested hours and years into comprehending the foundational materials of their field, so insights in them are less accessible to bystanders. Also textbooks tend to prioritize sharing all of the relevant information about a topic, instead of only the novel information or only the useful information.
News articles, by contrast, over-index on novelty, and are written for a target audience that seems expected to value entertainment and validation over technical precision.
I suspect that the process of distilling specialist knowledge into posts appealing to this expected reader is itself a good sieve for capturing which specialist insights lie in the intersection of novelty, explainability, and usefulness.
LW’s other edge over textbooks is timeliness of information—the lower expectations for a blog post vs a peer reviewed article allow faster publication and greater volume of candidate great posts, from which the community’s voting can then filter and highlight the posts that look great to the most people.
How did this end up working out?