Thing is, we simply don't know how big a reason it was.
Certainly it was a very conveniently timed shock, but unlike the bomb it wasn't a reason the Emperor cited for surrendering. And the threat the USSR represented to the home islands, especially Honshu, was limited, although I'll bet the way the Red Army tore into the IJA in Manchuria was convenient in helping to cow its hotheads.
I think the most we can say it that it might have been necessary, but it probably wasn't sufficient.
Certainly it was a very conveniently timed shock, but unlike the bomb it wasn't a reason the Emperor cited for surrendering. And the threat the USSR represented to the home islands, especially Honshu, was limited, although I'll bet the way the Red Army tore into the IJA in Manchuria was convenient in helping to cow its hotheads.
I think the most we can say it that it might have been necessary, but it probably wasn't sufficient.