Part of the (alleged) reason we dropped the bombs on Hiroshima and Nagasaki was to prevent the USSR from entering the pacific theater and muscling in on post-war Japan. I have to assume they’re referring to that.
Yes, but the plan was at most the invasion of Hokkaido, with the British and Americans invading Honshu, Kyushu, and Shikoku. Even that was considered an insane long shot since the Soviet Pacific Navy was unsure if it could undertake such an operation. Most attention was focused on Manchuria.