I don't think your interpretation about speed limits is correct compared to other factors mentioned in the nytimes article:
> more stringent enforcement of moving violations, revamping hundreds of street corners to slow down turning cars and rejiggering crossing signals to give pedestrians a head start.
For one, traffic in NYC just doesn't move that fast, especially on the streets where most of the accidents happened. It's not really appropriate to extrapolate NYC data to the rest of the United States unless you focus only on urban centers.
Second, almost all of the fatalities seem to happen around intersections. I'd expect to see more jay walking deaths in the years before the speed limit change if your interpretation was correct.