That's the same thing I heard for decades. I've always used a BP lube, usually of my own making, over the ball and in almost 50 years have never had a chain fire. Even so, to me it seems amazingly difficult for a spark to get past a swaged in ball without lube, then get past all that remain.

By the same token, unless a cap has fallen off, as tight as my caps fit, it seems it would be quite a feat for a spark to get past a cap and then all that remain.

I expect that's why I qualified my previous post with, "I don't know" and hope to never find out. All I know for a fact is they do occur.