It seems that we may have an inability to properly use SFCA so supplementing with them might not make much of a difference if we can't properly use or metabolize them.
I'm just pointing out that there's been a lot of real-life experimentation with microbiome-altering changes, with no significant effects. Thus it's not as simple as "boosting SCFA levels in the gut will reduce symptoms". If the problem is with transport or usage, then the levels in the gut shouldn't be significant.
The article does indicate that the gut/immune/brain interaction is far from simple. Studies such as this are identifying links no one knew about before. The altered SCFA levels might be a sign of immune dysfunction, rather than a cause. Some people do improve after microbiome changes, but since that's not common, it's more likely an individual variation in the whole system that allows it to respond to changes. My individual variation is making me overly react to fibre, but I haven't heard anyone else report that problem. Radical changes in my microbiome (flushing out due to food poisoning) seems to be responsible for curing my type IV food sensitivity, but again, it seems like a very rare problem.
I hope that this study gets proper followup studies. Learning about the interactions between the immune system(s), the brain, and the microbiome(s), and the rest of the body is important. With enough knowledge, they could replace some "smash the whole body" drugs with ones that target the actual problem. Also, these sorts of studies might lead to better diagnostics. "Patient has deficiency in isovaline production" is much more useful than "Patient complains about feeling fatigued."