Japan's holding of US securities is quite a bit more than China's. China's total holdings at last report is 235 billion in securities and they can't "call it in".
China's holdings are about 1/3 of Japans holdings who hold the largest amount. China represents about 18% of the total outstanding securities.
If they go to war with us, they won't bother telegraphing the fact by dumping their security holdings with the US. Would you rather their surplus dollars be supporting Germany, France or South Africa?
Communist china has already made it perfectly clear what their intentions are. Deflating the US dollar so we wont be able to buy anything from anybody else is not going to "telegraph" anything that they have not already said. Their missles are already pointed at american cities, they are building a war machine, they are going to war with Taiwan, which the US has pledged to ally with. We already know we are going to war with them, it is no secret.