Earlier quoted context omitted.
Another possible explanation is that some people commit multiple murders and being caught after a successful murder prevents the subsequent ones
> I'm basing this on the assumption that the average number of homicide incidents per murderer is somewhere around one
So it could very well be true that the average number of murders committed is close to 1 and at the same time the reduction is wholly accounted for by stopping multiple murders.
I still doubt that either cause speculated on in the article are the actual full reason, or even possibly the primary reasons, but it would at least be possible with the information provided.