Is it true that iOS devices use a hard coded time.apple.com dns for an ntp server?
Yes it's true and yes it is very hard coded.  Yes it uses standard port 123 UDP.

Jamf is the Apple Business Manager for iOS devices and there is no setting in there either.  See https://www.jamf.com/jamf-nation/discussions/11772/ntp-server-for-ipads

Like the article says create an A record for time.apple.com and set the IP to your NTP server and that's it.