nobody has been able to unearth this document
Yes, unfortunately, there is no direct evidence. There are only clues and hints: one letter mentioning French refusal of "something", some trails of German counteractions, some memoirs, changing foreign minister from pacifist one to more devoted one, the Ensign Incident in Gdańsk...
Dariusz Baliszewski wrote about it
Many writers write about Iran and most of it is absolute nonsense!
What does that have to do with anything?
The hypothesis about preventive action is quite plausible. And if you treat seriously diplomacy language it was at least to fly a kite if France wish to stop German remilitarization. The answer was negative and immediately after that new FM Beck started the equal distance policy with Germany and Soviets.
It's not a Michael Moore post-modernistic rubbish goulash or red-green gobbledygook on Man Made Great Global Warming.