Ya learn something everyday! I don't buy lard, so I have never looked. I cook in Bacon grease
My understanding is that any fat (hydrogenated or not, heated or not) are always subject to E-coli and other anaerobic bacteria, which in turn create hydrogen sulfide as a byproduct of their "anaerobic lifestyle" (smells like a bad rotten egg) Now if the Lard has absolutely zero water in it, it has been rendered completely, and if salt has been added, then yes the potential shelf life is next to forever. Once moisture is introduced anaerobic bacteria are free to thrive.
Cooking in oils (animal or vegetable) does cause "rancidity" without a doubt. The temperature of the smoke point gets lower and foods caramelize prematurely in rancid oil. Anyone that has gotten fries that are too brown on the outside and not quite done on the inside has experienced this. This isn't due to biological activity though, it is the oil becoming saturated with "other" hydrocarbons and organic material from being overused. When chefs speak of frying oil being "rancid" it is overused and the smoke point has diminished. For whatever reason that term has been used improperly traditionally in professional cooking. It is really that the oil has become exhausted, and doesn't cook cleanly. Now if you leave it sitting around at room temp, I don't doubt it will end up really rancid, with organic matter and moisture in the mix that's perfect for anaerobic bacteria.