the functional difference is that BCH doesn't have a meaningful portion of the SHA256 hashrate, so it's vulnerable to 51% attack all the time. But, it's basically worthless so nobody cares.
That's not a functional difference since it doesn't affect how the coin functions for users. BCH is certainly more at risk for a 51% attack but the risk is an abstract threat, not something that changes the fundamental nature of BCH. Your own logic actually demonstrates this because if some future event caused BCH to have a higher share of the hashrate then your own reasoning would demand that BTC be considered illegitimate without any technical qualities of BTC having to change.