Because if they keep eating a poor diet that encourages an unhealthy biome, they're going to defeat the point of a fecal transplant.
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..