This argument comes from Tsuyoshi Hasegawa's work, Racing the Enemy: Stalin, Truman, and the Surrender of Japan. I can't really claim expertise in this area, and have never read the book, but I have heard several professors speak highly of it, and I assume he is a reputable historian. Based on what I know of it, the argument makes sense- the nuclear bombs were no more destructive than many of the other bombing raids that had already been conducted (Tokyo, Yokohama), and the Japanese surrender came immediately after the Soviet invasion. Someone who knows more than me about it can comment more, I'm just highlighting the most reputable source, and what I believe is the origin of the argument.

I'd argue that the Soviet Union was the coup de grâce to any hopes the Japanese had of holding on for better terms.

Yes, the atomic bombs were shocking weapons, but keep this in mind - Hiroshima was bombed only 3 days before Nagaski, but Japanese surrender happened a full 6 days after. The Japanese didn't know how many bombs the US had, but it did already suffer a lot of damage from the fire bombings and still hadn't surrendered.

The Soviet entry into the war, with its invasion of Manchuria, is ridiculous in the pace and scale of how it took out Japanese forces. Japan took something like 80,000+ killed and over half a million were taken prisoner by the Soviet advance, which began on the 9th of August.

This is a controversial point, but people forget that Japan had the majority of its ground forces tied down fighting the Chinese for most of the war, and not on various islands in the Pacific. Those were troops that would've been used if the main islands had been invaded. Add on the post-WW2 rift aka the Cold War, and it's easy to see why contributions made by the Soviets (and to a similar extent, the Chinese once the nationalists lost and mainland China referred to the communists) were discounted while the American mythos was bolstered.