Out of curiosity, why do the eggs need to be cooked to the point of their yolks having the the same taste & mouthfeel of a model car rubber tire?...

Thanks. I will defer on the yolk comment, but as far as the "not enough salt" the recipe cited brings almost a tablespoon's worth of sodium to the equation, which proportionately is equal to the classic 1 cup of salt : 1 gallon of water. If the eggs are done in 1 cup of water, that's a classic 1:1 brine. Given that the egg shells have been, in effect, sterilized via the time spent boiling/simmering, I don't think bugs in the brine would be the issue, but rather the ability of the liquid to really penetrate.

@The Lard of Avon,
I'm gonna offer a few thoughts & theories about your questions.
1. The tea eggs, using this recipe or any other recipe, does not have "the same taste & mouthfeel of a model car rubber tire". Sure the whites are hard cooked, but it does not get any more rubbery when it is cooked this way. I've had many tea eggs; I can attest to this. The flavor is going to taste like the brine which is soy sauce-y, with hints of tea, cloves, anise, whatever spices you use. The only drawback of the long simmer is that the yolk does turn an unattractive shade of grayish-green, but the flavor and texture is still good. It's just a pukey-green color.

2. "Why can't you make the brine/marinade, if you will, and simply add perfectly cooked hard boiled eggs with cracked shells? "
I don't know the exact physics behind infusion, but from experience, you need a little heat or more time for the infusion of the brine to significantly penetrate the egg whites. With more time and no cooking (and we're talking days here), you run the risk of bad buggies growing in the brining solution. There's simply not enough salt in the brine to inhibit bacteria growth.