Dude, it's a simplified example, of course you can poke holes into it. Here, let me help you fill in the gaps:

    let mut object = prepare_generic_entry(general_settings);
    let mutable_msg = object.msg.get_mut();
    do_specific_message_modification(mutable_msg, special_settings);

The point is, that there are situations where you have exclusive access to a mutex, and in those situations you can safely access the protected data without having to lock the mutex.