does anybody know how could protonmail or anyone else ever be able to tell iCloud aliases?? it is impossible to register a new PM account using an iCloud alias

does anybody know how could protonmail or anyone else ever be able to tell iCloud aliases?? it is impossible to register a new PM account using an iCloud alias! I’ve always wondered, how? Especially now with the ability to hide your email on ios, what would be the use of that if there is no distinction? ¯\_(ツ)_/¯