That explains the German interest in heavy water?
Yes. American (well … Hungarian and Italian) physicists discovered that Graphite was a far better moderator. But, it had to be exceptionally pure graphite. They were getting it from Union Carbide. “You wouldn’t put Boron in your Graphite, would you?” Eventually they got the right stuff.
They kept this a secret, didn’t publish this information, and German scientists never figured that out, and therefore concentrated on heavy water as a moderator. They never got much farther than that.