I was thinking the same exact thing. Although Cornwell has no real proof since old Scotland Yard's records were destroyed during the London Blitz, her analysis of the case's circumstantial evidence that Sickert was the Ripper is most compelling.
Cornwell (like most Ripper enthusiasts) tends to skate inconventient facts. To fit her theosry that the Ripper was impotent, she declares Sickett was too, on the basis he had some kind of operation at the age of 5. Ignoring that the guy was married three times.