Hi Kiran,
Your findings are correct: for forms you create with Form Builder, for persistence, Orbeon Forms calls a REST API, and Orbeon Forms comes with implementations of this REST API for eXist, MySQL, and Oracle. For other databases, you would have to implement the REST API yourself. (Or this is something that Orbeon can do for you as a "sponsored feature", under a Dev Support plan, and the feature would then become par of the product.)
Alex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet