A control group of the same size doesn't work because the group of Americans in Havana working for or through the embassy had already been selected based on one criterion: They were the ones with symptoms who turned out to also have actual detectable, physiological brain damage. You would have to start with a control group as large as all Americans in Havana affiliated with the embassy because
1) Some of those developed symptoms and were then examined
2) Some of those who were examined then turned out to have physiological brand damage according to the MRIs.
So the question is if a randomly selected group with age groups corresponding to those at the U.S. embassy and the size of the original group would have a similar number of people with brain damage.
I had a neighbor who got very severe Alzheimer's in her early fifties. The same thing happened to an office worker at my high school. And as for hearing loss:
Some people don't realize until they are tested for other reasons, for instance, if they were the victims of a vicious microwave attack sounding like crickets!

But I am not sure if the ones whose only symptom was the loss of hearing were included in the final group.
I can no longer hear grasshoppers. Crickets are loud and clear.
ETA: It just occurs to me that if the 40 include people from other diplomatic missions than the Havana embassy, the consulate in Guangzhou, for instance, that would mean that the control group would have to be correspondingly bigger.