I'm not sure how reliable the Guardian is as a source but if this is true then it's horrific and this is just dementia alone. My granny has dementia and is in a nursing home near Birmingham and nobody was allowed to visit her obviously my Mother living in Ireland couldn't get over even after the visiting restrictions were lifted during the summer as she would have had to take 2 weeks unpaid leave from work to isolate when she got home but my uncle who lives a short walk from the nursing home wasn't allowed to visit. Granny has deteriorated alot due in some part to the isolation as dementia patients rely on family visits from familiar faces to keep them going.
Sadly mate, although the act of lockdown itself will have negative effects on people living with dementia, it's very unlikely ( nigh on impossible ) for that to cause 10,000 extra deaths.
It will undoubtebly cause some, and there's no doubt about the huge negative effect caused by being apart from family, but it's far more likely that those extra deaths were primarily due to people dying with undiagnosed COVID at a time when testing was rarely carried out on care home residents.
Most people in the later stages of dementia are frail and have suppressed immune systems and so are vulnerable to circulatory issues, respiratory problems and general viral infections, which is pretty much a perfect storm for COVID to affect.
If the excess deaths had mainly been caused by isolation then you'd expect to see the trend in excess dementia deaths continue, and the reality is that the opposite has happened, and less people than average are dying with dementia.
We need to do much better at making sure families and friends can keep in contact with those in care homes, because that loss of contact will definitely end up contributing to deaths, but I think that blaming that large excess in April on lockdown is just something that people ideologically opposed to lockdown either want to believe to support their argument, or worse, knowing it's not true, latch onto to use as emotional blackmail.