Ok I see that I completely misunderstood the nature of the Grand Jury in this case. I thought that the GJ met to review evidence and to determine if there was sufficient evidence to indict. If there was no indictment it meant that there was NOT sufficient evidence.

So, what does it actually mean in this case?

We don't know if the GJ decided to indict and the DA ruled against their recommendation, or if the GJ voted not to indict, or if there was a hung jury or sorts and the DA made the decision.