It uses both, MySQL for the metadata and ClickHouse for the logs. The selfhost page explains a bit more about the architecture.

edit: the connection to ClickHouse uses the MySQL driver, this is actually a very nice CH feature, you can connect to CH using the regular mysql or postgresql client tools. The PHP MySQL PDO driver works seamlessly. One catch, using advanced features like CH query timeouts requires a CTE function, check the model/txtlogrowdb.php file if you're interested.