That does not appear to be quite right. According to this source it was not an offer from the Japanese Supreme War Council. it was from some "peace party". In other words it was no a genuine offer. I am sure that the writers of it probably thought that it was, but they were not controlling the country:
What if Japan did offer nearly-full surrender terms to the US in early 1945? In the 1940s, just this was claimed — but it's not very likely.
blog.nuclearsecrecy.com
This source indicates that there was still resistance to surrender after the bombs were dropped:
"From the replies these diplomats received from Tokyo, the United States learned that anything Japan might agree to would not be a surrender so much as a "negotiated peace" involving numerous conditions. These conditions probably would require, at a minimum, that the Japanese home islands remain unoccupied by foreign forces and even allow Japan to retain some of its wartime conquests in East Asia. Many within the Japanese government were extremely reluctant to discuss any concessions, which would mean that a "negotiated peace" to them would only amount to little more than a truce where the Allies agreed to stop attacking Japan. After twelve years of Japanese military aggression against China and over three and one-half years of war with the United States (begun with the surprise attack on Pearl Harbor), American leaders were reluctant to accept anything less than a complete Japanese surrender.
Following the bombing of Hiroshima on August 6, 1945 (left), the Japanese government met to consider what to do next. The emperor had been urging since June that Japan find some way to end the war, but the Japanese Minister of War and the heads of both the Army and the Navy held to their position that Japan should wait and see if arbitration via the Soviet Union might still produce something less than a surrender. Military leaders also hoped that if they could hold out until the ground invasion of Japan began, they would be able to inflict so many casualties on the Allies that Japan still might win some sort of negotiated settlement. Next came the virtually simultaneous arrival of news of the Soviet declaration of war on Japan of August 8, 1945, and the atomic bombing of Nagasaki of the following day. Another Imperial Council was held the night of August 9-10, and this time the vote on surrender was a tie, 3-to-3. For the first time in a generation, the emperor (right) stepped forward from his normally ceremonial-only role and personally broke the tie, ordering Japan to surrender. On August 10, 1945, Japan offered to surrender to the Allies, the only condition being that the emperor be allowed to remain the nominal head of state. "
They were still holding out hope to hold on to at least some of the countries that they had conquered, nor were they will to have any oversight. The bombings appear to have been legit. Even in hindsight.