Seems to use tls-sni-01. This is my guess by looking at the certbot-nginx source directory: https://github.com/certbot/certbot/tree/b1826d657ffc7c278041...
Certbot can use different plugins for validating the name and for installing the certificate.
You can configure HTTP-01 to work and use "certbot -a webroot -i nginx -w /path/to/whatever -d example.com -d www.example.com".
https://community.letsencrypt.org/t/solution-client-with-the...