They mention it, but a big factor driving this is almost certainly obesity. I'd really like to see this done with proper controls for obesity, age demographics, smoking, etc. I think there'd be interesting stuff to learn from that. It's shocking how difficult it is to do these studies correctly. E.g. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3674138
Australia has similar obesity rates to USA, and dramatically lower COVID mortality.
E.g if obesity is > 30 BMI, Australia might have similar percentage to the US, but US might still have much mor people > 35 BMI than Australia which could have mostly 30-35 BMI people.