Indeed, obesity is a disease. I'm not sure why this paper group obesity as a lifestyle choice of smoking, lack of exercise, etc. So thank you for pointing this out.

One study showed that obesity is not a risk factor among those older than 60 (only those younger than 60 years) https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa415/5818333

But another study showed otherwise, that BMI is an independent risk factor (smaller sample size): https://pubmed.ncbi.nlm.nih.gov/32271993/

A third study I found showed that, after adjusting for confounds, BMI is no longer a risk factor. But their decision tree analysis showed that age is the most important factor, followed by BMI.

https://www.medrxiv.org/content/10.1101/2020.04.08.20057794v1.full.pdf+html

Overall, it does seem that obesity is more concerning for those aged <60. For those over 60, other risk factors are more important.