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some small human studies, people who ate a protein-restricted diet for several weeks lost weight and fat mass, and saw improvements in their fasting blood sugar—even though people on these diets tended to eat more calories.

Fgf21 will do that, but is it a long term benefit in humans? We have other ways of reducing human blood sugars while eating more calories (keto).

The actual papart they are talking about https://doi.org/10.1016/j.cpblue.2026.100079

Conversely, a high-protein, low-carbohydrate diet was recently shown to be less effective than a moderate-protein diet in lowering blood glucose, improving glucose tolerance, and promoting prediabetes remission.[38]

And if you follow reference 38 you will see low carb is 45% energy from carbs.... No where near a fat based metabolism, which is what most people will think from "low carb"

These seemingly contradictory studies leave many wondering what the optimal amount of dietary protein is for healthy aging

It's not contradictory, the epidemiology they use and the one off 45% energy from glucose study they call low carb - don't control for underlying metabolism. Yes in the context of a high glucose diet consuming low protein will activate fgf21 increasing metabolic rates by 20ish% showing blood glucose improvements... But that isn't the only lever, and quite frankly if you want better control of blood glucose maybe a glucose free diet should be considered by the authors.