Certainly, it can keep time without a phone, and it's probably good at it. But it just seems a bit funky to call an Apple Watch an "exceptionally accurate timepiece", when it can cheat using the phone to check a remote UTC every few hours. I'd be curious to see how well it keeps time over a longer period in Airplane Mode.
Particularly when some high-end quartz and mechanical watches lose seconds per year with *no* external input, the conventional standard for "exceptionally accurate", this is an entirely different battle, and I balk at the characterization.
It's kind of like the difference between the guy who can multiply 4 digit numbers in his head, and the guy who can do it with a calculator. It's the same result, but the situation and methods make it much more meaningful and impressive.