In my experience, blameless postmortems can be really useful for the process of getting to the bottom of things and creating a plan of action, but can also enable people who are reckless or incompetent, and make it difficult to ensure that the people who really need to learn something from their failures have actually learned something.

As applied to this situation, it doesn't really matter. The Chinese government has a vested interest in any investigation being labeled inconclusive. Whether it was a lab leak or a result of unhealthy agricultural practices, China doesn't want egg on its face. They'll do everything in their power (which is a lot) to avoid having the finger definitively pointed at them.