It's really not that hard for the cops to track down someone in China. Besides, clowwindy didn't take any precautions before. I believe clowwindy once revealed the company he worked for.
Cops can do a lot of things, but they don't pull information from thin air. Github would be one source of information.
It's odd that this entire, and very popular, discussion on HN doesn't delve into how this individual was found.
It's also interesting that your reply is from a new account with only this comment.
Again, how the Github user was actually located by Chinese police was not disclosed or even discussed here. It's interesting that mentioning this has resulted in two comments by new accounts, solely to blame Chinese authorities as discovering the user on their own, with no evidence for it.