Node.js neither. It merely passes the request to PHP-FPM. Neat hack but I fail to take it seriously. I don't see the benefit. Maybe I missed something.
Node.js is low-level enough to do some protocol things fast enough PHP can't. I can imagine building a load balancer or proxy to PHP. Yes, it's probably not the best solution, but it could be done.
In this case Node.js acts as the webserver as Nginx or Apache would.