Not true. In the end, Truman did not demand unconditional surrender from the Japanese. Even after the A Bombs, they insisted that they insisted on keeping the emperor (as they had before then). Truman thought about it and then overruled his pro-FDR advisors who wanted him to hold out for Unconditinal surrender. He agreed to the Japanese stipulation, hence it was very much a conditional surrender. In any case, Truman's decision on this point was not part of some nation building "master plan." In fact, it was a bitter pill for more pro US New Dealers who regarded Hirohito as the anti-Christ.
BTW, even pre-WWII the Japanese emperor's power was highly limited, though he did provide a useful rubber stamp for the militarists.
That was a back-room diplomatic deal -- it was still officially, publicly, an unconditional surrender. By not hanging the emperor, Truman allowed the Japanese to save a little face, and gained a pliant figurehead who would be helpful in rebuilding and reforming Japan.