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)
Correlation is not causation.
Ahh, but which way does the relationship go..?
Unsurprisingly if I'm dying of something really serious and lying in bed sick as a dog.....
...I lose muscle mass.