"Of course, I'm thinking that there may have been "something" in/on the vessel that was the real agent."
Leeks and garlic, even today's varieties, contain sulphur compounds that are known to have strong antibacterial properties. Copper also has anti-microbial properties... I've heard of copper sulphate preparations being used to treat minor skin infections.
Why they become so much more potent when left to stew? That's interesting.