“It is known that Chinese people are smart.” 中的It 指代什么?

“It is known that Chinese people are smart.” 中的It 指代that Chinese people are smart.it 不是只能指代一个名词吗？SAT作文中让这么用吗？it 不只是指代一个名词。it可以指代天气、距离和时间；it可以做形式主语；it可以做形式宾语。
“It is known that Chinese people are smart.” 中的It作形式主语， 指代that Chinese people are smart这个真正的主语。