We talked a bit about the state of physics in 1940 in my 'Modern Physics' (aka Physics 401 aka nuclear/quantum). Looking backwards through time its easy to imagine they were planting disinformation or trying to throw off people from the potential there, but that view is mostly bolstered by knowing what is possible now. Had it come out in 1942 after 'pile 1' was demonstrating fission then yes, it would be subterfuge.
But perhaps most amazing are these results from physics experiments in 1940 and then the changes in the world by 1945. Such profound changes are also likely when someone figures out how to tap the energy from fusion as well.
There was some experimental evidence (and open publication) by 1939, to boot. The threat outlined by the Einstein-Szilard letter is that the Germans must be aware of the idea and might be developing a weapon so planting disinformation was probably not high on anyone's list of priorities.
But perhaps most amazing are these results from physics experiments in 1940 and then the changes in the world by 1945. Such profound changes are also likely when someone figures out how to tap the energy from fusion as well.