It would only be circumstantial evidence. A man's semen could get on a prostitute's shawl because he raped and murdered her. But it could also get there because he was one of her johns.
I heard this detail on the radio story of the same topic: The shawl did not belong to the victim, but I forget the details of how they could know that. They also traced the fibers of the shawl to polish or russian origins, which would be consistent with belonging to the polish emigrant.
Oh I'm sure it would, but without multiple pieces of evidence linking him to multiple scenes, it wouldn't be enough. Even if multiples were found, it wouldn't be enough if he had any alibis. Finding a piece of clothing with semen stains on a prostitute isn't exactly surprising, after all.
A similar case is actually taking place now in Italy. They found a few droplets of blood from a middle-aged family man on the underwear of a 13 year old girl that was killed. Even though it's blood, all this proves is that he had some kind of (inappropriate) contact with the child. On its own, it's not enough evidence to convict him of murder. Now imagine a much more explainable scenario like semen on a prostitute, and you realize how weak the evidence is if not corroborated by further evidence.
Well, one would assume that the police work of the time was done to the most reasonable extent possible and if the outcome of that police work was him as a suspect, being able to confirm DNA at the murder scene would only serve to affirm that suspicion. It all depends on what other evidence police based their suspicion at the time. The person who bought the shawl said he consulted the original written police reports in the archives. I suppose anyone could do the same, or... buy the book. ;)