This was my thought. To add on, any dietary change that has major impacts on a diabetic's blood sugar levels, specifically if they are taking insulin, can be life threatening. Taking the wrong insulin dose with low blood sugar can and does kill people.
Also and probably more to the front of the lawyers mind who reviewed this paper prior to publication. Cause of death may be difficult to determine and if you engaged in this fasting and then happened to die even if fasting was not the cause of your death it would be likely your family would have grounds to sue. Even if the publisher won the lawsuit he would be out legal fees, thus a notice like this is prudent on almost any actionable medical information.
