Yes, that's the way its supposed to work. You can only change styles with media rules. To make other changes (like create new elements or change text) that has to be done on the "Master Page". Toggle between Master Page and Media Rules in the Responsive panel.
For example, if your text changes with Media Rules, then create both text elements and with media rules just change the Visibility of them to hide & show the correct text.
Hope that helps,
Kent
Google Web Designer team