Watched the 4 part documentary on the case, The Ripper, on Netflix. It underlines how the lack of skepticism about the tape and the letters diverted the investigation away from listening to women who had been attacked and survived. As is mentioned, anyone with a passing knowledge of the Jack the Ripper deaths would find the letters so reminiscent of the letters sent then, which were also almost certainly hoaxes, to be very suspicious of them. The tape also sounds in tone very similar to that approach.
The documentary is very well done. The testimony from those who survived attacks quite astounding.