Frailty comes from a lack of bone density, not lack of muscle.

Also, you must not have read the article because the second paragraph describes subjecting them to regular fasting regimes, not lifelong caloric deficit. I don't believe lifelong caloric deficit is even possible nor is it mentioned in the article. Lastly, you can gain muscle and then maintain it on a caloric deficit anyways, it's what body builders do every season. So even your initial assertion is wrong. Now I know that is a little different, but you can build muscle and not gain weight too. I feel like you're way over simplifying the process of muscle gain and retention and the essence of this article is that obesity is bad