Can you share your entire config.xml?
It's also possible that the setting is working correctly -- the preference only affects the webview itself. If your app has a scrollable element that uses native scrolling, that container can still bounce. Depending on how you've built your app, it's possible that would mean your entire app would still bounce -- that is, if your navigation bars and such were within a single containing element with scrolling enabled. As such, it might help to see your index.html or your app's HTML structure.