But clearly, diet, the gut microbiome, the immune system, at the very least, are linked, and linked tightly. So it would not be surprising at all, that there's a microbiome component. But we don't know what it is; how it affects the underlying disorder, and how to approach it for treatment as of now.
And as you alluded to with "genetic risk factor"; its well possible that though there is a component of the microbiome involved in the disease; it may not be significant enough to act as a vector for treatment. Or that it is just a comorbidity. We just don't know!
Why not? That would be a fast way to find out if it works.
That doesn't work (the treating depression part), but suddenly, this person becomes more susceptible to obesity / develops diabetes.
These things must not be rushed into!
I don't think the authors are drawing a casual effect here; in effect, this is scientific anecdata: hey, we tried this and this happened. We think you should know.
I think it's great they published the result, even if the n=1.
Assuming this is due to the fecal transplant, it just goes to show, we are messing with a major functional system in the body. And we should tread very very carefully. Scary..