As the article mentions, it seems more likely that the decreasing death rate is due to higher rates of young people being infected vs older people. Also due to

As the article mentions, it seems more likely that the decreasing death rate is due to higher rates of young people being infected vs older people. Also due to increased testing catching the less severe cases. Back in April here in the U.K even healthcare workers weren’t getting tested. Now you can get a test posted to your house for free if you show even the mildest symptoms, so much more of these mild cases are getting counted. Also, anecdotally, older people are still taking precautions to minimise contact with others whereas younger people are basically going about life as normal. Hence, the virus is spreading around younger people much more than older people. For example, the U.K had a huge number of deaths in care homes earlier in the pandemic but I believe most care homes are still locked down to some extent while at the same time the pubs are open and kids are heading back to school next week.