That one is easy to answer: if you get a node running code from before the BTC/BCH split, which chain would it follow? AFAIK, that old node would follow the BTC side, since BCH changed the proof-of-work parameters in a way that wouldn't be accepted by older nodes, while BTC only changed some validation parameters in a way that would still be accepted by older nodes (the older nodes might accept some things which newer nodes would reject, but the older nodes would not reject anything newer nodes would accept, so the current BTC chain should still be seen as valid by older nodes).