One under director of the Foreign Affairs office posted it on Twitter, a site blocked in China. By this logic, the US is also telling their own people that the virus is a biological weapon because one senator said it on Fox News.

"Mr. Zhao’s remarks were spread on China’s most prominent social media platform, Weibo, under a hashtag: #ZhaoLijianPostedFiveTweetsinaRowQuestioningAmerica. By late afternoon on Friday, that hashtag had been viewed more than 160 million times, along with screenshots of the original Twitter posts."

Do you find the US government responsible for a conspiracy theory being popular on social media?

Did a US official start it on a platform controlled by the US... then yeah maybe.