The author clearly means professional publishers, who have editors and fact-checkers. Self-published books already lack trust. The reply also misses several other points the author makes, which I find ironic because it kind of goes into the direction the author bemoans: The author wrote a longer article to lay out his thoughts and it sure took him time to write and any reader time to read and digest and here is a quick oneliner as a rebuttal that took no time and effort and is superficial.
Do publishers really have fact-checkers? My understanding was that support for authors is now relatively minimal, even for established authors, and no one really has the time or resources to second-guess everything an author has claimed. I take as a key example Naomi Wolf learning after her book was "done" that a significant chunk of it was based on a misunderstanding of an admittedly confusing 19th century British legal phrase. https://nymag.com/intelligencer/2019/05/naomi-wolfs-book-cor...
I think maybe the idea that a single author spending months or years on their research, which the publish as a single bound and polished work is misguided -- an academic trying to do similar work in multiple articles would have gotten review from peers on each article, and hopefully have not spent so much time working under a correctable misunderstanding.
Fact checking as a separate job is more for journalism than books. But editors have fact checking as part of their jobs. (It is not copy-editing, which is a different job.)
Many nonfiction authors will hire a fact checker separately. They don't want to look like they missed something. Errors still happen, of course.