This is very much not the case. Bomb theory and critical mass come from a fundamental understanding of neutron chain reactions. You need to have good data on fi

This is very much not the case. Bomb theory and critical mass come from a fundamental understanding of neutron chain reactions. You need to have good data on fission properties and neutron mean-free-path distances, but that requires only extremely tiny amounts of Plutonium or natural Uranium, it does not require enriched Uranium. Other than that the other critical factor is coming up with the correct model/theory for nuclear weapons. Something that the Japanese got right, the Americans and British got right, and the Germans (specifically Heisenberg) ended up getting extremely wrong. Fine tuning critical mass calculations was important in optimizing the bomb later on but it wasn't necessary to developing the basic underlying theory of how bombs work or of pointing the way toward implosion assembly and Plutonium breeding.