It would be interesting with what the carbs were substituted. As far as we know most sources of carbs are really bad for you. Some even worse than plain sugar, because they raise your blood sugar so much in a short time.

Of course fats (and proteins) are necessary to be healthy, because without them you will die. If you don't provide enough carbs to burn the body will metabolize its own though hence the usefulness of external carbs is questionable except for calories. That's the whole idea behind low carb and keto diets.

It would be really interesting to see this study repeated with another demographic. At least I assume that the normal diet is very carb heavy for most people there which might skew the results in different ways.

It was an interesting read nonetheless and I hope there will be more studies on this topic.