The argument for Heisenberg being intentionally incompetent is that he made two "incorrect" choices in which path to follow.
He selected heavy water as the reactor moderator, even though it is very unusual and requires a big plant to make it.
He selected plutonium as the fissile material, even though it doesn't occur in nature and have to be created in nuclear piles.
The argument is that Heisenberg, being a highly intelligent man, would not have made those mistakes, and that he therefore intentionally made those choices to delay or stop Germany from getting the bomb.
However, this ignores that Heisenberg was a scientist, not an industrialist. I think Heisenberg simply chose the options he thought had the greatest chance of succeeding. He didn't think Uranium was a good candidate for a chain reaction, and that it would be easier to make a bomb out of Plutonium. He also thought it would be easier to create a nuclear pile if it was moderated by heavy water.
He was definitely correct about the heavy water. Making a heavy water reactor is easier than a graphite or ordinary water reactor, and in addition, a heavy water reactor can work with un-enriched uranium. This means that although you need facilities to create heavy water, you don't need facilities to enrich the uranium, to some extent compensating for the need of heavy water.
And it's hard to claim that he was wrong with regards to Plutonium. He rather was "too correct". The Manhattan project decided to pursue both Plutonium and Uranium bombs, and during the development of those bombs they had to come up with a more complex bomb design for the Plutonium bomb, because Plutonium turned out to be so reactive that it would probably explode to early in the process, making it likely to "Fizzle" if it had been used with the original simpler design, while the simple design could be retained for the Uranium bomb.
So I think he rather chose the options that was the easiest for him. The "safe bet", so to speak. He probably either underestimated the time and effort required in creating enough heavy water and then enough plutonium to build a bomb, or he didn't make his choices with regards to the time frame at all, but based them entirely on what he thought would be easiest, as opposed to what would be quickest.