While it has been suggested that Werner Heisenberg was foot dragging or even possibly deliberately sabotaging their research, I don't think going in the direction of a heavy water reactor is an example of this.
That contraption would have worked. It would have worked as well as Femi's graphite reactor anyway.
More importantly it would have given them good research answers that they would need to either breed Plutonium, or build Isotope separators.
The big secret at the time was that chain reaction fission was even possible.
That is not what the biographer of Werner Heisenberg says. Rather, he says they just didn’t know, and that Heisenberg didn’t begin to have the resources he needed to test the heavy water anyway.