The author doesn’t mention whether iCloud Keychain is enabled or not. When iCloud Keychain is enabled, this is the expected behaviour - internet accounts get synced to keychain.

Probably what happened is that iCloud Keychain became enabled (I wouldn’t be surprised it enabled itself by default on a fresh install), and the accounts got synced. Unlikely to be malicious. Also, I’m pretty sure Keychain is end-to-end encrypted, so Apple doesn’t _technically_ have your email addresses.