On some people's phones IPv4 addresses can't be reached. Their carrier doesn't bother moving IPv4 over a network that doesn't need IPv4. It does translation at the edge, and the iPhone is OK with that.

Apple's requirement is that even though you know your server is definitely 10.20.30.40† on the public network, and you hate IPv6 you must not hard code 10.20.30.40 inside the app and ship that to the App Store.

Once you reluctantly change it to a DNS name ten-twenty-thirty-forty.fuck-off-apple.example that resolves to 10.20.30.40 - Apple allows that.

Because now when your app is used on some IPv6-only carrier network, the carrier goes "ten-twenty-thirty-forty.fuck-off-apple.example ?" and it gets 10.20.30.40 and it says that's an IPv4 address, don't use those around here, and it adds a translator step, it gives the phone an IPv6 address for ten-twenty-thirty-forty.fuck-off-apple.example and the phone connects to that address, which is a translator that connects to 10.20.30.40 on the IPv4 Internet.

This stuff happens all the time and you don't notice. But if Apple allowed app vendors to just scribble IPv4 addresses inside their app software it would break.

† No that isn't a public IP address. It's an example.