Again, this does NOT address my question, which is whether Mail.app is required to use iCloud custom domains.
One of the pages you referred me to has a section for setting up iCloud Mail on a Windows computer, which implies that Mail.app is NOT required for iCloud Mail.
Moreover, this Apple support article gives the technical settings for connecting any email client to iCloud Mail. It concludes with guidance using an “app-specific password” that refers to “any other third-party mail client” thereby implying again that Mail.app is NOT required for iCloud Mail.
So, again, is there anything about iCloud custom domains that requires the use of Mail.app?