As I understand, this protocol is used between an iPhone and iCloud and it being open or not doesn't change anything because there is no alternative iCloud or iPhone.
You’re mistaken, you don’t only connect to your iCloud from iPhones.
You connect from any compatible client; and the effort that has gone in to the Mail client for iOS means it’s a decent enough mail client for non-iCloud mail accounts too.
Apples closed ecosystem is mostly its developer tooling and iMessage.
