It's interesting that nearly all of these pieces of advice are fairly non-controversial. What's the point of posting stuff anonymously if it's the same stuff that most people would be happy to attach their name to?
But one thing we learned in the process was that seriously successful people doing their thing feel pretty gagged.
It's not worth it for them to write even things that seem uncontroversial to you or me, because someone out there might hate it, or misinterpret it, and start creating problems for them.
Why not do something actually anonymous, though? Put up a form box on your website where people can write anything in response to the prompt, and let folks access it through Tor? Then post the results with the appropriate disclaimer. You might need to moderate it a bit to cut out crap like "PENIS1!!11!1!", but you might also get some genuinely interesting commentary.
Then again, this is basically what Blind does.
As for why that's the case... well most people/sites don't want to become a social pariah, most underrated creators are by definition unpopular enough most haven't heard of them and truly unpopular opinions are unpopular enough that they're unlikely to get posted.
Still, when I heard 'career advice you wouldn't want your name associated with', I was thinking it'd be something like the stuff in that James Damore memo or what not. Or something inspired by Machiavelli's The Prince.