This is an example of the law being out of date with technology. When these laws were drafted, there were no smartwatches on the market (at least not in any significant numbers).
The question that needs to be address is whether the person was using the watch with his hands on the steering wheel. If he was, then his appeal has some merit, because effectively, he was using it as a hands-free device.
By the way, I'm the happy owner of an Apple Watch, and it is genuinely not a gimmick. I used to have to take my iPhone to the gym to listen to music and enter stats on my running app.
Now, I can leave my iPhone in the office and listen to music which is loaded onto my Watch via bluetooth headphones.
And the maps solution is well thought out too. I was meeting friends at a bar I'd never been to last night, so when I came out of the tube, i simply looked up the post code, and let the watch guide me there without having to constantly whip my phone out to check directions (the watch taps you one way to turn right, and another to turn left until you reach your destination).
I think of the watch like those steering wheel remote controls for car stereos. You really don't need them, because for decades, we were all perfectly able to use the stereo controls on the unit...steering wheel controls are simply a nicer way to do this.