That's been a speculation for decades. Here's a more technical analysis of Heisenberg's calculation of critical mass.[1] He had a figure of tons of uranium. In reality, the bare-sphere critical mass of U-235 is 52Kg. The Hiroshima bomb, which was a very simple design, used 62Kg. With reflectors and compression, smaller amounts can be used.

Whether Heisenberg miscalculated this by intent or mistake remains unknown.

[1] https://www.mpiwg-berlin.mpg.de/sites/default/files/p467_3.p...