Heisenberg used a bad approximation that led him to incorrectly believe he would need tons of U235 to produce a bomb. His model for bomb viability was essentially "a random neutron walk should not leave the core until producing enough generations to explode." In reality, you don't need that much U235 because earlier generations have exponentially more descendants and they are closer to the core -- the later generations can leak neutrons out of the core like a sieve and the exponential party deep in the core won't even notice, effortlessly replacing them by cooking a tiny fraction of a generation longer.

It was Peierls and Frisch, two Jewish German scientists, who figured out those dynamics, modeled them with the Neutron Diffusion equation, and repeatedly prodded UK intelligence until they were taken seriously. Then the project got sent across the Atlantic for safety and scale-up and the rest is history.

Germany could have had the Atom Bomb in WWII, but Nazi Germany could not have because Jewish Science gave a decisive advantage.