The diesel emissions revelations make me put less faith in the argument, "There is no way so many people could keep shenanigans so great secret so long."
The fact we managed to invent, create, and detonate nuclear weapons without anyone finding out is why I believe secrets can be kept.
There were quite a few scientists feeding information to the USSR, some under the belief no one nation should have this power. Nuclear weapons weren't that secret.
The USSR was still technically an ally when the first were dropped. It would be interesting if Japan or Germany was in the know (and they also likely were, just musing)
They were only allies of convenience and everyone knew that. They were allied to Hitler up until he invaded them.