What caused the Japanese surrender in WW2? (Revisited)

After the post I wrote on whether the atomic bomb or the Soviet entrance into the war caused the Japanese surrender, I would be remiss if I didn't point out this article on how the Soviets were passing along allied battle plans to the Japanese in the hopes of prolonging the war in the Pacific.  They were doing this with the hope that if Japan stayed in the war long enough, they would be able to invade the northern islands of Japan and claim a piece of the nation.  Despite this recent news, I still question whether or not the Soviets had the logistical ability to carry out such an invasion of the Japanese home islands.