That may work for domestic public opinion, but getting any international sanctions to stick on China would require a lot more than circumstantial "they won't le

That may work for domestic public opinion, but getting any international sanctions to stick on China would require a lot more than circumstantial "they won't let us investigate" evidence. International "law" (if such a thing truly exists in practice) is wonky, and rarely based on nuanced views or established facts. Even domestically, that above quote only applies in specific cases - civil cases, or criminal cases where it can be proven evidence was actually destroyed. I'm not personally aware of clear evidence of bad-faith destruction of evidence right now on China's part. China's refusal to allow a third-party investigation would then be more similar to pleading the 5th, something even US courts are prevented from punishing.