HERE MIGHT BE A SPOILER OR TWO
A few things. For those who haven't read it, spoiler alert:
One, "Jenny" wasn't Nemo's daughter either.
Two, Geschwitz pulls out a gun and is in a crazy state. My feeling here is that Macheath came in looking for tail, but thought maybe they were trying to rob & kill him(Lulu as bait--it's not an unusual mugging tactic), and took revenge as a gangster would. Keep in mind, Lulu and Geschwitz were probably speaking German and he couldn't understand. It doesn't overtly say--in fact denies at the end--that he's JTR. It just puts him in the character's place in this scene. (In my Lulu, he's not going to be JTR either, but mine is in the present day)
Also, Macheath here is not feared or disliked by the people at the docks(look at the ending where he dances with Polly Peachum). My feeling is that this murder was unusual for him. Particularly given it turns out it's the 14th Earl of Gurney who was Jack the Ripper.
And yes, THAT character only thought he was JTR and was hallucinating that he was in 1888. But I still like the reference. And given Norton's time travel, even that might fit in. And besides, remermber: at one time or another, EVERYone's been thought to be the Ripper, and this fits with that.