"Some mice were placed on calorie-limited diets, another group followed intermittent fasting regimens, and others were allowed to eat freely." Sounds like only some of the mice were fasted intermittently, others had calories restricted. I'm certainly open to the benefits of intermittent fasting and have experimented with it myself. Having practiced (natural) bodybuilding for years in the past I was never able to maintain muscle mass whilst restricting calories. This may just speak to my lack of talent as a bodybuilder. Pros are typically still using some sort of exogenous testosterone derivative while dieting for competition so I don't think that's a good example. As a competitive weightlifter in multiple weight classes for over 15 years I feel I have a good understanding of how my body reacts to various dietary interventions and in my experience anything but brief caloric restriction leads to loss of both fat and muscle (not to mention strength). BTW the Cambridge Dictionary's definition of frail seems to cover both our usages.

https://www.nature.com/articles/s41586-024-08026-3#Sec9 I'd recommend reading the actual paper not the article