[opo]

The Soviets could never have done a large invasion of Japan. They had a few ships that the USA had given them as part of Project Hula, but that is nothing compared to what would be needed for a full scale invasion of Japan. They did have plans to possibly attack Hokkaido, but as the wikipedia entry says "Historians have generally considered it unlikely that an invasion of Hokkaido would have succeeded."

https://en.wikipedia.org/wiki/Proposed_Soviet_invasion_of_Ho...

In comparison, the proposed allied invasion was planned to have 42 aircraft carriers, 24 battleships, and 400 destroyers and destroyer escorts. Even that wasn't considered enough:

>...Ken Nichols, the District Engineer of the Manhattan Engineer District, wrote that at the beginning of August 1945, "[p]lanning for the invasion of the main Japanese home islands had reached its final stages, and if the landings actually took place, we might supply about fifteen atomic bombs to support the troops."

https://en.wikipedia.org/wiki/Operation_Downfall

[bell-cot]

Japan's decision to surrender in Aug'45 was based on when the Japanese knew then, not on what historians know now. And several previous Japanese conclusions of "the USSR will not be able to do X" had proven catastrophically wrong. For example - https://en.wikipedia.org/wiki/Soviet_invasion_of_Manchuria#B...

[opo]

>Japan's decision to surrender in Aug'45 was based on when the Japanese knew then, not on what historians know now.

Nobody is saying anything different.

>...And several previous Japanese conclusions of "the USSR will not be able to do X" had proven catastrophically wrong. For example …

The Japanese had already moved all of their experienced troops from Manchuria before the invasion. They were surprised that the USSR would break the Soviet–Japanese Neutrality Pact, but the defense of the home islands was their main concern at that point.

As your source says:

>...The Soviet entry into this theater of the war and the defeat of the Kwantung Army were significant factors in the Japanese government's decision to surrender unconditionally on 15 August, as it became apparent that the Soviet Union had no intention of acting as a third party in negotiating an end of the war on conditional terms.

The Japanese knew the USSR was not a threat to the main islands and the USSR knew they would likely fail if they tried to invade Hokkaido. The Japanese had hopes that the USSR would be willing to negotiate with the Allies on their behalf, but once the Soviets declared war was, they knew that would not happen.