If a person is severly demented, you couldn't see delirium. However, on top of a progressive dementia, it is possible to have a delirious state. You could wax and wane and become much worse. If we took the drug away, the delirous state would disappear. But the underlying dementia and its progression would still be there.
Dementia tends to be a slow progressive illness, different than an acute event.
Delirium is a metabolic, reversible state. Dementia is not reversible. You could develop a dementia due to a metabolic disturbance, for example, thiamine deficiency. And you could see a halt to the progression of the dementia if you administed thiamine. But you would not be able to recover the neurons you lost.