A couple of things worth knowing:
It's a review/opinion article, not a new experimental study. It surveys existing evidence that dietary protein regulates health and longevity across species, and that restricting protein or specific amino acids has emerged as a potential strategy to promote healthy aging.  So it's synthesizing prior research rather than presenting a single result.

The strongest evidence is in animals, not humans. In rodents, cutting dietary protein from about 20% to 5–10% of calories increases maximum lifespan by over 20%, and restricting methionine alone extends rodent lifespan substantially. But evidence for lifespan extension in non-human primates remains limited and is better characterized as improvements in healthspan rather than confirmed survival benefits. The jump from mouse to human is exactly where skeptics push back — mechanistic and animal data don't reliably translate.
The review itself admits the amino-acid story is messy. On leucine, for example, human leucine supplementation increases muscle protein synthesis but does not improve muscle function, and while leucine restriction increases fat mass in Lamming's own mouse work, other groups found the opposite — that leucine restriction reduces fat mass and improves glucose control. So even within the paper, results conflict

Treat the press headline ("eat less protein, live longer") as the weakest version of the claim