"arguing that fear of Soviet invasion persuaded the Japanese to opt for surrender to the Americans, who they believed would treat them more generously than the Soviets."
Well you can't argue with this statement. The soviets would have treated them far worse. However, according to the Japanese - it was the bomb that drove them to surrender. Additionally, though the Soviets were right next door, the U.S. had Japan essentially blockaded and completely surrounded. We would never had allowed the Soviets to close in and conquer Japan; nor did the Soviets have the ability to mount an invasion of mainland Japan. This blockade of Japan, and the fact that their forces in Asia were essentially cut-off, led to the weakening and thus capitulation of those Japanese forces when it came time for the Soviets to attack. The Soviets essentially attacked a weaken and already beaten Japanese army in Asia - a small feat, not a great decisive one.