I think you are considering “involving only natural [meaning found in nature] sources” as a criteria for “healthy” which is not really necessary. Healthy is about the result of health (physical, emotional, spiritual, whatever a particular person may consider) when discussing a diet, and while for some ‘natural’ may make them feel psychologically better, I don’t think that’s what most are considering (I certainly am not, nor are all the people that take supplements on any diet at all or as prescribed by a doctor, or who are ready to eat 3D printed meats and other extremely artificial foods).
If my body ends up having no biomarkers of disease, is robust, looks great, allows me to accomplish all the tasks I desire, can continue for what I would consider a substantial lifespan and beyond that of others on different diets, and keeps me feeling great and avoids sickness, I’d likely consider that healthy in a strict physical sense. If it required eating humans immediately after killing them, on the other hand, I might consider it unhealthy in a psychological, spiritual or moral sense, but I can’t really argue with the idea that it’s perhaps healthy in the physical health sense if the results show that to be the case.
There may be reasons to avoid even the “healthiest diet in the world”, and different people can raise different objections to it, but that doesn’t stop it from being the healthiest diet in the world if the objection is anything other than on the grounds of the results on health of the dieter.
But, perhaps to be fair, health should be defined and the standard for measurement needs to be understood if we really want to come to a conclusion of what is healthy vs not healthy vs healthiest, and I don’t think that’s been established yet (I’ve indicated my idea above, but that certainly won’t agree with everyone’s idea).