The JDBC drivers (not only ours) are specific to a certain database type. So Elasticsearch-SQL JDBC driver will "talk" to an Elasticsearch cluster only. It's like an interface that translates SQL queries to Elasticsearch queries.
If you'd like to test Elasticsearch-SQL, you could just send REST request (using curl for example) to Elasticsearch using the SQL API. Alternatively, if you'd like just to play with some data without setting up Elasticsearch yourself, you could start with https://demo.elastic.co. There is some preloaded data in there and from the main page you could click on Query it button from the Elasticsearch SQL box in the welcome page.