"It has NOTHING to do with chain failure."
Of course it has to do with failure. Its not the only cause or probably even the most common one; but chain elongation can certainly contribute. It is a symptom of pins and bushings being worn away (i.e. pins get smaller and bushing openings get bigger) just as the OP described his pin being worn away; as that happens, pins and plates are more likely to work themselves free from eachother; particularly if , as in this case, the chain is old so is stiffer generally than a newer chain. True, lots of thigs can contribute to chain failing, one of those things is chain wear. So in this case I would surmise that wear of the pin combined with other factors stemming from the chain's age combined to cause the failure.