No of course we could not walk around in the USSR. But we had more access then, than we do now with the PRC. Even beyond their Communist systems, the Chinese are innately the most secretive nation on Earth. They've been that way for a very long time.
Building missiles is not all that hard to conceal. They are built indoors, not in open shipyards. It is even easier to conceal it if you don't final assemble them and instead, kit the parts. They can be assembled rapidly, using demand flow manufacturing (e.g. Kanban system). Pardon the pun, but it's not rocket science.
More telltales than you think on missile production and deployment.
There won't be any overnight fielding of 500 ICBMs.
I'm not dismissing the Chinese, but I don't give them mythical abilities either.