Unnecessary for China, against someone whose family was within their grasp. That does not mean it wouldn't be a very convenient tool.
You'll note that China found it less troublesome to threaten someones family with violence than to risk a diplomatic incident by violating the integrity of a diplomatic mission.
Well the US is not exactly a country to get on the wrong side of. It's pretty clear the US never really went to bat for the guy either, not wanting to be on the wrong side of China. Britain and Ecuador have no need to maintain good relations so it's an entirely different kettle of fish.
You'll note that China found it less troublesome to threaten someones family with violence than to risk a diplomatic incident by violating the integrity of a diplomatic mission.