Your comprehension seems spot on to me, though maybe you feel like you're missing something given the apparent discrepancy between the hype vs. value of this research. I think there are two relevant factors here:

1) MIT is very good at publicity and press.

2) Causal neuroscience experiments are still technically challenging to implement in complex behaving organisms (e.g. monkeys and humans). We have methods such as microstimulation (invasive) and transcranial magnetic stimulation (non-invasive), but these methods are pretty ham-fisted. They indiscriminately activate (or deactivate) all the neurons in and passing through a given volume in the brain (optogenetics is promising in this regard, but it's still a nascent technology in monkeys). So while this study may appear weak because it's correlational, it may actually represent a meaningful step forward in the field.