I _strongly_ suspect the iCloud web login will block brute force attempts. What I do wonder though, is if there's some other place an iCloud/AppleID login can be brute forced without appropriate rate limiting? Maybe an IAP API endpoint? Or an in app advertising endpoint? I wonder if the "check whether an IAP succeeded" API that the "just redirect you dns to my server and add my root cert" "exploit" uses is failing to block brute force attempts?

Even if iCloud allowed 10 failed logins before locking out for an hour, every hour, every day for seven years, that would still only let you crack a 4 digit lowercase-alphanumeric password. I'd be willing to bet that either the attacker took the password from something or that 'alphanumeric' is a misleadingly good description for the password.