There are so many things wrong with what they are stating and how it works:
Hormones and cortisol changing gill cell count - even if this was true, what does gill tissue cell count have to do with osmotic pressure control?
Reacts oppositely to control osmotic pressure - So by adding some electrolytes to the water, you are able to reverse the laws of physics? Sorry, but osmotic pressure is osmotic pressure, regardless of the solute.
Magical water seldom affected by ammonia - At lower pH, there is an abundance of H+ ions, which form ammonium ions, a non-toxic ion of ammonia. At pH of 7, ammonia concentrations will be just as toxic, unless there is some endless amount of prime in the "magic water"
Cortisol produced when fish placed in magical water - cortisol is a naturally produced hormone that comes from the adrenal gland. Animals produce it when they are under stress. It enables the body to suppress an immune response in high-stress situations as a survival mechanism...think fight or flight. Similar to the affects of adrenaline. So, sure, if you stress the fish out badly enough, it will produce cortisol, enabling it to ignore the physical and chemical results of the stressful situation, presumably enabling the fish to survive long enough to make it to more suitable conditions. The problem is, this is not an indefinite stasis in the body...eventually there will be a tipping point.
Anything claiming that causing undue stress is a great way to invoke a survival method, which enables keeping animals in completely improper conditions...well lets just say its not for me.