These studies are generally longitudinal studies across a number of years, where greater muscle mass or strength early in the study correlates with lower all-cause mortality across the duration of the study. It's not looking at survival rates for acute afflictions.

Examples:

* https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6427792/

* https://pubmed.ncbi.nlm.nih.gov/12242311/

* https://pubmed.ncbi.nlm.nih.gov/10795731/

* https://pubmed.ncbi.nlm.nih.gov/11984288/

(There are many, many more of these, and they all seem to agree)