The information within the article you have quoted is incorrect and not based on any factual scientific and/or experiential evidence with which I have had access to for the last 30 years, AND for the following reasons........
The body will use FAT as its main source of sustenance on a water-only fast (ketosis) and where in addition, the body will also use non-essential elements within the body as fuel; as part of "protein conservation", the body begins seeking out all non-body-protein sources for fuel: nonessential cellular masses such as fibroid tumors and degenerative tissues, bacteria, viruses, or any other compounds in the body that can be used for fuel. This is part of the reason that fasting produces the kind of health effects it does. Also, during this period of heightened ketosis the body is in a similar state as the one that occurs during sleep - a rest and detoxification cycle. It begins to focus on the removal of toxins from the body and the healing and regeneration of damaged tissues and organs.
Over a 30 day Water Fast a person generally loses a maximum of 1-2 pounds of muscle mass, as the body will naturally conserve its vital resources, and why as the fast progresses, the faster will gradually lose less weight per day than at the beginning or in the intermediate stages of the fast.
The body economizes its use of food reserves (including protein usage) in its own inimitable and innate way, as a measure supplied by Nature to maximize the length of time in use of food reserves and preserve the integrity of the vital organs. In other words........to prolong life for as long as is possible.
There is absolutely no danger whatsoever from the body living from its essential proteins such as cardiac muscle as this represents starvation (a destructive and death-dealing process) and not fasting (a constructive and health-enhancing process): the demarcation point between the two is the genuine return of hunger: I've been there, done it, seen it, and actually experienced it with no danger to my health but only distinct benefit.
Muscle protein (or any bodily protein) can only be replaced by consuming amino acids, which will then constitute human protein: no amount of exercise will build protein within the body, but it will build muscle-mass which is an entirely different concept.
There is however a greater need for protein by the body post-fast, as witnessed by Dr Shelton & Dr Tilden and others, but this is supplied by eating greater quantities of protein rich foods, which contain the essential and non-essential amino acids needed for repair and maintenance of the body.
Overeating after the breaking of a fast is not recommended by anyone within the field of Natural Hygiene, and where this would indeed build excess fat, but the recommendation is NOT to overeat and consume only those foods which conform to the physiology of the body: that of a high water content, as the body is composed of about 68% to 70% water anyway.
Do you personally know of anyone who is protein deficient? or even HEARD of anyone who is protein deficient?
No. Neither have I.
(EDIT) Two further thoughts on your post Hamo..........
To build muscle mass you will need to do weight-bearing exercises (as you have said) and where this can be done with progressively heavier hand-weights whilst rebounding or even isometric exercises which produce results more quickly.
The "three weight loss programs per year" which you have said theoretically results in the loss of more protein than would otherwise be the case is simply not accurate.
Body composition depends entirely on what you feed it: carbs', proteins, etc etc and where an exercise regime after or in between these programmes will build or maintain muscle mass. This is why weight-lifters consume special protein (amino acid) foods to help build more muscle.