The article argues that as long as Stalin was neutral, the Japanese had some hope of negotiating a better surrender. After Stalin declared war, Japan was out of options and was forced to surrender. Therefore Stalin was the deciding factor in surrender.
The atomic bomb was not the deciding factor for two reasons: 1) the decision to surrender happened before the impact of the atomic bombs was fully assessed, and
2) the effect of the bombs was no worse than the massive conventional bombing that Japan had already experienced. The vast majority of Japan's large cities had already been pretty much destroyed by the time the atomic bomb was used: it was a "raindrop in a hurricane."
The atomic bomb was not the deciding factor for two reasons: 1) the decision to surrender happened before the impact of the atomic bombs was fully assessed, and 2) the effect of the bombs was no worse than the massive conventional bombing that Japan had already experienced. The vast majority of Japan's large cities had already been pretty much destroyed by the time the atomic bomb was used: it was a "raindrop in a hurricane."