The position is much as though they had got hold of an incriminating document, but couldn't read it because it was in cipher but we know that the draft treaty wasn't in cipher couldn't be in the nature of things so that won't wash.
Draft treaties are always written in cipher so that they are impossible to read. 
mnli hypothesis: Draft treaties are always written in cipher so that they are impossible to read.  premise: The position is much as though they had got hold of an incriminating document, but couldn't read it because it was in cipher but we know that the draft treaty wasn't in cipher couldn't be in the nature of things so that won't wash.