Do we know that for sure? I don't know that much about Alzheimer's in particular, but for other forms of dementia people frequently have periods where they are more lucid and can access memories and other periods where they are less functional. This would suggest that for at least some forms of dementia most of the actual memories are intact but inaccessible. Alzheimer's does involve the direct destruction of synapses which suggests that the information has been completely removed in those synapses, but it is hard to tell how much information is being actually destroyed and how much is being rendered inaccessible.