There are a large number of Western people in China for business reasons, and major Western countries have diplomatic offices throughout China. The US has a Consulate General post in Wuhan itself.

That gives the West an independent source of information about what is going on in China. China can still fudge the numbers, but those independent observations place a limit on how far China can fudge without getting caught.

I don't think you have to trust their numbers, but could they hide extensive outbreaks like the OP is talking about (all numbers being roughly the same and measures having little observable effect)? It would be something like 4 million+ excess deaths.

It should be noted that this is the same CCP that until recently have been simultaneously accusing Spain and Italy and the US of being the origin of the pandemic outbreak, instead of Wuhan.