If the licensing stuff from Google is not successfully preventing piracy for you, then the backup functionality offered from Google is not going to work for that purpose either.
There is absolutely zero reason to believe the backup functionality is more secure (or even as secure) than the licensing check.
You can just check it for yourself. Use a rooted phone (or use the emulator) and check out for yourself if you can still access through the linux file system what you're backing up through the Android manifest. And yes, you won't have any problem doing so (as long as you use the emulator or use another rooted device).