That's something that Soviets very much want the world to believe, but they have never had shown how they could even possibly get their troops anywhere near Japanese mainland, what with Russian/Soviet/Russian navy historically being more dangerous to people who serve there than any enemy. And whatever happened in Manchuria or Korea didn't really concern Japan by that point anyway.
There were no either Japanese Navy nor Japanese Air Force at this point, so quality of Soviet Navy does not matter. All they would need to do is to transport enough bodies to the islands.
Oh, they had enough troops for Manchuria, but of course at that time what happened there wasn't all that important to the mainland, and USSR had little in the way of ability to get there anyway.
What Zhukov had shown at Khalkhin Gol, just as against Germans, was that with a significant advantage in manpower and equipment (double the manpower, 7 times the tanks) Soviets can actually beat up someone.
