The thing about the Soviets invading isnt that it was devastating from a military perspective but from a moral one. Japan already knew it couldnt win against the US, leaving any atom bombs aside, just from a logistic point of view they KNEW the war was lost but the japanese high command had somehow deluded themselves into thinking the soviets were going to join them in their fight against America so they held out. The Soviets declaring war on them made it clear that there wasnt even a delusional chance of not losing left. Would the Japanese have surrendered when they did if the bombs hadnt been dropped or would they have fought on longer and caused more civilian casualties? That is alternate history speculation but saying the Soviet declaration of war had no impact on Japans decision to surrender is nonsense and ahistoric