This has been the Soviet line since 1945. Of course, Stalin only entered the war because he saw that Japan’s defeat was imminent and wanted to grab as much territory as possible.
The likely fact is that the combination of disasters pushed Hirohito to decide for peace.
Russia actually worked to prevent the end of the war so they could enter (and claim Japanese territory).
The Japanese govt had contacted the Russians to act as intermediaries between themselves and the allies about ending the war. The Russins told the Japanese they would do so, then did no such thing. Instead, they make preperations to enter the war against Japan on the date they had promised the other allies.
Which also points out that the Japanese didn’t fear Russia entering the war, because the Japanese thought the Russians were helping them negotiate an end to the war.
As I understood it at the time, Truman had for some time been trying without success to get Stalin to declare war on the Japanese.
With the dropping of the atomic bombs, Stalin did as you say; he decided to get in while he still could.
It was cause and effect. Without the Hiroshima and Nagasaki bombs, Stalin would have continued to play cat and mouse with Truman. But he saw the end and acted.