After all, knowledge of how to make the stuff was in the public domain, and in the open scientific literature, having been synthesized more than seventy(70) years earlier.
It had been re-synthesized in 1939 and patented by the Swiss firm Ciegy who proceeded to offer patent licenses to everyone : neutral, Allied and Axis nation alike : just as the Swiss firm Oerlikon did with its anti-aircraft guns, used by almost all military forces during WWII.
But in wartime, busy generals and even busier bureaucrats and politicians don't have time to read scientific journals, patent applications and industry magazines : though they do like to glance through their familiar newspaper from time to time, to relax.
An "open" secret can still be effectively a total secret
So if you can keep news of a weapon out of the newspapers, as the US successfully did by censoring both DDT's domestic use and domestic coverage of DDT's success overseas, you can keep it effectively secret - albeit an "open" secret - throughout the war.
Unbelievable but true.
And an example of how the Allies planned to keep synthetic penicillin an effective - if open - secret as well....