I don't think China would intentionally spread COVID as a weapon, and not necessarily because they are so nice.

Simply,

• COVID, for a very long time, spread in China mostly. Who wants to attack themselves?
• COVID has a pretty low risk of death for a weapon
• It's risky as fuck. Imagine if people find out, and say the US goes to war about it (the US had started wars for much smaller things in the past). That war's not gonna be pleasant to say the least.

So, It just sounds like a stupid thing to do.