China has routinely been opaque, blocking investigations over all sorts of things. I don't like it either but I think it's premature to use it as evidence of malice on China's part.
China (and everyone else) doesn't get to have their cake an eat it too. They want to be opaque as national policy, that's their prerogative, but they don't get to do so and expect positive inference regarding what is behind the curtain.
In other words, assuming the worst plausible scenario is the only, or at least a, rational interpretation of possibilities against a policy of intentional opacity.
Can you see how a government might be concerned about keeping an adversary do that because of the sovereignty violation it brings up as well as concerns about spies being part of that group (eg how the CIA used a vaccination program as cover to find Bin Laden in Pakistan)?
It’s not the only rational explanation. It’s just the one you choose to prefer (and it might perhaps be the most plausible given how the Chinese government appears to operate, but it’s not beyond reasonable doubt). Heck, this was the theory behind invading Iraq and looks like the US never did find those WMDs.