A colonel was originally a "column" commander. That is the highest ranking officer who will command "pure" columns of say, infantry, cavalry, or artillery.* That would be opposed to a general (or general officer), who is someone who commands "mixed" or combined arms units. In Qian's case, his "column" was rocketry, and he was at the top of his specialty, which is why he was made a colonel, and not some other rank. On the other hand, Leslie Groves who directed the Manhattan Project, was (initially) a (brigadier) general.
The U.S. military will from time to time employ people from outside to fill specialist roles that are deemed "mission critical." This includes foreigners and even former enemies. For instance, Wernher von Braun himself was employed first by the U.S. Army, and later by NASA after World War II. In Qian's case, it helped that he was a citizen of an allied country, China, and that his appointment was temporary, as Schwern pointed out.
*This would be for American, and other non- western European usage, as noted in the link in the first sentence. A commenter pointed out that by about 1800 CE, the western (and in some cases central) European usage was for "columns" to be larger bodies of soldiers commanded by the American equivalent of Major Generals, or what the Americans would call "division" commanders.