The answer as I understand it, is that anyone who's continuously running a node won't be confused by such a fork, but someone who just joined the network has no way of telling which fork is the true one.
Put another way, if you know the true chain at time T, you also know it at time T+1. But if you're just joining the network, or you went offline for a bit, that means you don't know which chain is the true one and need some outside-of-protocol way of determining which fork to trust.
Put another way, if you know the true chain at time T, you also know it at time T+1. But if you're just joining the network, or you went offline for a bit, that means you don't know which chain is the true one and need some outside-of-protocol way of determining which fork to trust.