bm wrote:
You'd be surprised how common it is for a front derailleur to "rub" the inside of the crankarm. A front derailleur commonly has only about 2 to 3 mm of clearance from the outside of the cage to the inside of the crankarm when adjusted correctly. If it's adjusted just slightly off so that the lower edge of the cage is angled outward a tiny bit, it can rub the crank arm. It can be so slight that you don't hear it or feel it when pedaling, but after several hundred thousand pedal revolutions, that tiny little bit can score the inside of the crankarm. Whether that is enough to cause the crank to break, who knows, I just noted that the location of the break in the OP was about at that same location.
Did you not see the photo of the broken crank? I doubt that bike has been maintained even slightly. Crank was probably damaged in a crash or falling off the bike rack or something and the damage never noticed under the layers of grime.