They did a stupid ass job them

They probably didn't expect their president to be idiotic enough to be against vaccines and recommend stuff like bleach.

They're a developed nation and could've had better control.
They were able to hinder Chinese trade a bit.

And didn't they try and fail to kill Castro in many stupid ass ways?

I think them doing it is more likely than China doing it to their own, considering the trade issues that the virus caused for China at the time.

And again, they were the first to use nuclear weapons on civillians. Did it twice even.
I don't think they'd have an qualms about using biological weapons, if they thought that it'd give them an edge.

They also can float the 'China virus' conspiracy to confuse the public and make them anti-China too.