Earlier quoted context omitted.
PHP is usually deployed in a FastCGI multiprocess setup, so the server will keep handling requests on the other processes while the hashing process is blocked. I wonder whether threading really saves you, given that there are only so many cores in a system and password hashing is CPU heavy. Naively it seems a DoS would involve sending as many parallel requests as there are cores, which is not a lot and can easily be…
The blog post that HN is reading is served by PHP-FPM + nginx, without any extra special features to handle the load of an unexpected "oh hey we made it to the front page of HN". It's also hosted on a relatively cheap VPS.
For things like blog posts, I'm sad that Movable Type fell out of favor. The pattern of using dynamic server pages to edit blog posts and generate static files for your viewers just makes so much sense to me. Then again, these days we can host our single-page JS front-ends on S3 + CloudFront and hook them up to API gateway and Lambda / Beanstalk talking to RDS and "not have any servers" :)