There are perhaps some missing pieces here.
- He was a temporary colonel.
- Non-citizens are allowed to serve in the US military.
- China was a US ally.
He was a Chinese citizen living and working in the US with a doctorate from Caltech. He was hailed as a genius, and his work in jet propulsion made him very valuable to the US war effort.
Qian Xuesen was twenty-four years old in 1935, a fresh graduate of Shanghai Jiaotong University, when he used a scholarship to get to M.I.T. A year later, he moved to Caltech to earn his doctorate, and Theodore von Karman, a legendary Caltech professor, pronounced Qian an “undisputed genius.” When the U.S. went to war, he joined American scientists in the study of jet propulsion, and helped produce technology to counter German rockets. Then he joined the Manhattan Project.
Source: New Yorker, The Two Lives of Qian Xuesen
Mr. Qian served on the United States government’s Science Advisory Board during World War II. On the war front in Germany, he advised the Army on ballistic-missile guidance technology. At the war’s end, holding the temporary rank of lieutenant colonel, he debriefed Nazi scientists, including Werner von Braun, and was sent to analyze Hitler’s V-2 rocket facilities.
Source: NY Times, Qian Xuesen, Father of China’s Space Program, Dies at 98
My speculation is his job of debriefing Nazi scientists and inspecting German rocket facilities would involve moving around Europe and working with the US military, and that would be easier if he held the rank of Colonel.
Tragically, after Communists took over China he was accused of being a Communist on the thinnest of evidence and deported. He would go on to develop China's rocket industry.