Not allowing investigation is not necessary conspiracy to cover up known evidence. It might just be an admission that there is a nonzero chance there is evidence there... and if the world finds that, there could be dire consequences if people decide China should be held liable.
Glasnost is not standard policy in China and they have very strong reasons not to change that for reasons which have nothing to do with what did or didn't happen in the Wuhan lab. In the unlikely event the CCCP has complete certainty about what happened which completely exonerates the lab, they still wouldn't want to release anything other than controlled media briefings. If you start allowing international observers in every time you say something wasn't your fault, people can draw some much stronger inferences when you don't...
(Not to mention that there's little reason to believe the higher reaches of the CCCP know the answer to the question of whether there's any evidence any better than the average person. They're not virologists, and if evidence of lab malpractice exists the virologists aren't exactly incentivised to incriminate themselves by sharing it with them)
It’s not just stonewalling an investigation. It’s outright destruction of files and documents so that an investigation literally cannot happen. Wiping servers, destroying emails, shredding files, burning samples. These are things that don’t necessarily implicate you in guilt, but make you look guilty.
It’s just kind of rich for people to grandstand and say “there’s no evidence to support this theory” when the evidence that may support it has been systematically destroyed. There is enough circumstantial evidence to warrant a search, and if this lab situation had unfolded in the US the lab would have been searched.
