Basically the lesson is, if you’re providing a valuable service to your users, it doesn’t matter how your website looks.
If users leave your site because of how it looks, then you’re not serving them. If users go to your site in spite of not looking terribly good, then you’re providing a valuable service (and that service is not “web design”).
Craiglist absolutely gets praised because they have been so successful with that design. There are plenty of websites that looked like Craigslist but were not as successful, and you won't see them in thought leadership blog posts in 2020.
Also, Craiglist has evolved their site quite a bit and at this point their "design" is no longer a matter of simplicity, but essentially a visual aesthetic that they maintain as a matter of branding and culture, kind of like Amazon giving people door-desks long after it became cheaper for them to just buy regular office furniture in bulk.
Plus, Craigslist couldn't just change their design to something "modern", it'd put people off - people don't like change. Or, people don't like sudden change.
It reminds me of a national adverts website over here, Marktplaats; it used to be a very 90's, table-driven lists of links kind of deal with a fugly brown background. But it took off nevertheless, and became a major business that was eventually taken over by Ebay (in 2004). It's very slowly changed its front-end to a more modern facing one. But they made sure to do it slowly. Evolution rather than revolution.
But I still think the post you're responding to is too cynical (shocking on HN, I know). One author on one website praised Berkshire Hathaway's website. So what? Plenty of people on plenty of other websites praise mediocre things created by non-rich people all the time.
Craig Newmark is a billionaire, granted not on the same level as Warren Buffett, but having your "not rich at all" threshold between those two seems not that useful.
If users leave your site because of how it looks, then you’re not serving them. If users go to your site in spite of not looking terribly good, then you’re providing a valuable service (and that service is not “web design”).