I just can't help but notice a flaw in the internal logic of that book summary.
On the one hand we are told that the question "When was it written?" will be finally answered.
We are also told that the diary team is "confident it is a genuine Victorian document".
Well if they've answered the question "When was it written?" they must know if it was a genuine Victorian document or not. Why are they only "confident" and not "certain"?
But if they HAVE answered the question and are confident that it is a genuine Victorian document then QED, unless they are holding paradoxical views, the diary must have been written before the death of Queen Victoria.
Then we are told:
"The diary is either a genuine document written circa 1888/89 or it is a modern fake. There is no other feasible option."
As they have already established that the document is Victorian then the answer must be that it was written circa 1888/89.
Given that they've established this, it must be regarded as a bit of a shame that the questions: "Was the diary written by a serial killer? If it was, was he Jack the Ripper?" don't receive definitive answers. So near yet so far, it seems.