It says in the article that it's a 'unique, random address', so I don't know where you're getting the 'combination of the user's appleid and the target app/webs

It says in the article that it's a 'unique, random address', so I don't know where you're getting the 'combination of the user's appleid and the target app/website' from. The purpose of the service is the same as other temporary email services, to anonymize signing up for services by providing a fake email.