It is known that the bacteria that produce high amount of butyrate are beneficial, e.g. by decreasing the risk of colon cancer, but this does not seem sufficient to explain the increase in muscle strength that seems to be caused by this species alone.

The study has first found in humans a correlation between muscle strength and the presence of this bacterium. Then they have attempted to determine whether this correlation is due to a causal relationship by killing the gut bacteria in mice, then feeding them with this kind of human gut bacteria. The result was an increased muscle strength, which seems to confirm causality.

How the bacteria increase muscle strength remains unknown. I think that a possible explanation may be that this bacterium produces some substance that mimics a human hormone, e.g. a steroid, in which case it would be a kind of natural doping.

Natural or otherwise, one must wonder if there are similar downsides.