Our guess is that this is most likely just an act of lip service. There is always a loophole. You can still use the voice messaging function to just tell the driver you're willing to tip. Or, if you're able to type in Chinese, you can hit the 消话 and type the amount your willing to pay (e.g. "加10快"). Either way, it's highly unlikely your cabbie will turn down the offer of a little extra cash.
In related news, Didi Dache also added a new "Zhuan Che" service to the app a few months ago to compete with Uber. I've used this to take an A6 to work a few times, for double the regular fare.
Interestingly, you can still add a fee when you need one of these, but only 10rmb, 30rmb, or 50rmb. Think of this as a luxury black taxi, as these are all private drivers. Most are nicer cars like Audis and Toyotas, but there's also some average cars. You can see the car model and the driver's stars on the screen when you order it, and if you don't like the car you can cancel. Note, you will need some basic Chinese reading skills to use this function.
For more info on installing and using Didi Dache, read our article on Chinese apps. And you can always call 96822 or 62580000 to get a cab directly from the taxi company.
But here's some good news in the world of cabs. The government is going to start cracking down on those cabs that hide the backseat safety belts. For anyone who has tried to find the seat belt in the backseat, only to have the driver tell them "oh it's there" when it's clearly buried deep under the seat, this is a small victory.