Yes, I understand the correlation does not equal causation.
I trust you understand that the gold standard double blinded RCT could not be done here for obvious ethical issues.
There are plenty of mechanism studies that propose potential causal reasons.
Specifically damage seems to be happening to the hippocampus. I will gladly post references if you are interested.
There was also a 2012 systematic review noting a positive relationship between IQ and fluoride in water.
doi: 10.1289/ehp.1104912