They’re the most reliable source we’re going to get without being party insiders. There’s still Soviet-style inflation of figures to meet quotas but China has been cracking down on that for the last few decades because they want accurate data for the five year plans. I think it’s more of a problem with outer provinces, less so for the major manufacturing hubs.
Alternative sources to verify are a bit harder to find without knowing the languages (lots of the NRDC and NBS stats are available in English).
Yes, people also compare some of these statistics with export/import data and with data from other countries on the other side of these transactions, and the numbers match.
Are those statistics reliable?
In the US there are often good alternative sources for data: the discussions about unemployment numbers have been interesting (e.g. after private ADP numbers released). https://seekingalpha.com/article/4850656-jobs-data-from-alte...
The lies in the Soviet 5 year production stats were relentlessly mocked in 1984.