Only a small point but why would this be grounds to dismiss the letter? Surely the actual Ripper could have used them in the same way the fake one did.The letters were examined at the time and it was shown that they incorrectly recorded the number of victims at the time they were written. One body hadn't been found and hence wasn't known about and that everything in the letters was already in the public domain. The police ignored the analysis showing this. It was also shown that the letters used many phrases from the original ripper letters from the Victorian case, some of the language used wasn't still commonly used at the time of the Yorkshire ripper. There's a Netflix documentary that details all of this.
Amongst all the other ridiculously incompetent things they got up to, this seems irrelevant.