I think this is a nice example of how myths emerge. There are two parts to it: genesis and perpetuation.
Genesis: someone observe that (1) Agfa papers have a high silver content and (2) Agfa papers don't have expiration dates. They conclude that these characteristics are related. Note that no evidence is offered of this supposed causality, so we can't really assume it holds true.
Perpetuation: someone recounts something they've heard long ago. They may or may not have full recollection of all the relevant details, but the gist of the argument remains (at least, we assume it does...) Whether they themselves actually believe it or not, they offer the information they obtained in a matter-of-fact way: this is what I've heard, make of that what you will. Then, someone else comes along, remembers it and recounts it...etc.
Nobody in this chain is lying, purposefully trying to distort reality or otherwise play tricks on the rest of us. It's for all we know all in good faith, and why shouldn't it?
Before you know it, internet lore has it that silver-rich papers last longer. That silver-rich films have better tonality or somehow 'hold more tones' or whathaveyou.
Before you know it, people like Bob Shanebrook have gotten so scarce that nobody is in a position anymore to separate the wheat from the chaff.
PS: among amateur photographers I know, the story is that the longevity of papers like Portriga derives from the presence of cadmium. Make of that what you will. I don't know and have never looked into the matter, but I have no reason to doubt that cadmium was used in the manufacture of papers. AFAIK that's an established fact. What I do not know is why it was there, and I don't expect that it was included to make the paper last longer. I honestly don't know if it had that (unintended) effect.