This feels like very motivated reasoning. Yes, the Soviets took Manchuria, but they had no capacity to invade Japan in 1945. They had a very small navy to begin with, and essentially no pacific fleet at all. This is important because you need a huge fleet to launch a full scale invasion of an island country.

For instance, the invasion of Okinawa Island alone, which is extremely small in comparison to Japan, involved 1,300 U.S. ships and dozens more from the Royal Navy.