> This opens up the possibility that the bacterium under investigation could be used as a probiotic to help preserve muscle strength during aging

Maybe, but it's really hard to control for other variables here. They don't know what's causing this bacteria to diminish over time in older adults in the first place.

It could totally just be dietary habits getting worse over time as people let themselves go. Regardless of age, most people already don't eat enough protein and when they do they might not be getting "complete" proteins either (missing amino acids is common with plant-based foods).