As long as you use DNS instead of hardcoding IPv4 addresses (and if you are using AWS's ELB, you are using DNS instead of hardcoded IPv4 addresses), a mobile phone in an IPv6-only network will connect to your backend as if it had a IPv6 address, with the mobile network doing all the translation.
That's probably why Apple is adding the requirement: no matter whether your backend is on IPv4 or IPv6, your client must be able to connect to it as if it were on IPv6. Due to IPv4 exhaustion and the costs of CGN, the more phones can use exclusively IPv6 (even if through NAT64 or 464XLAT), the better for the network operators.
As long as you use DNS instead of hardcoding IPv4 addresses (and if you are using AWS's ELB, you are using DNS instead of hardcoded IPv4 addresses), a mobile phone in an IPv6-only network will connect to your backend as if it had a IPv6 address, with the mobile network doing all the translation.
That's probably why Apple is adding the requirement: no matter whether your backend is on IPv4 or IPv6, your client must be able to connect to it as if it were on IPv6. Due to IPv4 exhaustion and the costs of CGN, the more phones can use exclusively IPv6 (even if through NAT64 or 464XLAT), the better for the network operators.