She appeared to me to be arguing that if you become distracted and go an do something else, it's just ordinary memory loss, but if it's something weird you do instead, it's dementia.
However, I'd characterise the different thinking with the glass as being essentially a tendency towards paranoid delusions, and the underlying process still being the same; i.e. in terms of memory, I don't see a difference between something like going to make a pot of tea and, say, noticing the flowers need watering and doing it, and going to make a pot of tea and noticing there's been an intruder and putting the evidence somewhere safe.
It seems to me to be more a problem with things like the ability to reason and connections with reality. Someone with, say, a brief reactive psychosis or a longer disorder with paranoid features might react the same way, and someone with dementia might never do so.
The notion of putting the glass somewhere safe as evidence is, to me, totally logical. Putting it in a cupboard with other glasses might be poor memory or poor reasoning.
So I don't think it's actually a very good example and it could mislead people without clear caveats.
In other words, problems relating to paranoia are different from problems relating to memory and whilst they're more common in some forms of dementia, they're less common in others.