Dementia isn't a single disease its a symptom or a general term which covers a wide range of things. There are many causes of dementia. You can treat some causes and not others. Saying someone has Dementia loosely speaking is like saying someone has a cough. Both Dementia and a cough are symptoms that can be caused by a wide variety of things. At least that's how I understand it. So its very possible you read someone developed a drug for one cause of Dementia and not the others.
EDIT * Before the usual suspects misread my post/ I am not saying having Dementia is like having a cough. The cough is just an example of how Dementia is a symptom that like a cough can be caused by a great many differing things. So there is no single cure for Dementia. This is why some people get treatment and others do not.
There is no cure full stop. Some mitigating drugs which slow it to some extent but that is it.