The undelying point is mentioned early in the article:
> The traditional solution to blocking nuisance crawlers is to use a combination of rate limiting and CAPTCHAs. The CAPTCHA forces vistors to solve a problem designed to be very difficult for computers but trivial for humans. This isn’t perfect of course, we can debate the accessibility tradeoffs and weaknesses, but conceptually the idea makes some sense.
> Anubis – confusingly – inverts this idea. It insists visitors solve a problem trivial for computers, but impossible for humans.
Fundementally, the idea that PoW is a good way to tell humans from bots just doesn't work.
Captchas, rate limiting, authentication, etc are all part of the solution.
The more bespoke a captcha solution is, the less likely that bots, especially the kind of low effort bots that ignore rate limits and hammer sites, will have the ability to break it.
Arguably, anubis has a much better harm-vs-protection ratio at much lower difficulty setting where it functions less as a PoE system and more as an obscure way to block lowe effort bots. Of course, the more it gets adopted, the less well this will work.