Because the incident isn't about deflection to US.
It was retaliating and warning US against pushing Wuhan conspiracy theory (Tom Cotton) and calling covid19 Wuhan virus (Pompeo) after WHO re-designation. Both these events proceeded the Chinese tweets.
Current US administration has consistently been the instigator of bellicose diplomacy. But people conveniently forget when it comes to China who has been remarkably patient compared to other countries, including US allies.
US is pushing crazy propaganda to deflect blame from their inadequate response. In return, China unleashed one of their mid tier diplomats to troll crazy US rhetoric coming from a Senator and Secretary of State to demonstrate that China is willing to escalate to dirty propaganda games. Hence why it had to be de-escalated by Chinese ambassador to US, a diplomat 2 levels higher than the original foreign ministry spokes person. It also plays well to Chinese domestic audience.
If China wanted to deflect blame, there would be much more plausible ways. The fact that China chose to confront US conspiracy with Chinese conspiracy should be interpreted as signalling to not go full retard. The message and messenger was calibrated for this affect. And what do you know the US backed off on this avenue of attack and has shifted to boosting Taiwan. Also concurrent to the conspiracy tit-for-tat was banning each others journalists. These events have to be evaluated in proper context and timelines.