Maybe hidemyemail allows easily creating many accounts on a site. And some big site didn't like it.
The "Sign-up via Apple" button and creating an iCloud email yourself have a slightly higher barrier than creating a new throwaway hidemyemail email (1 API call w/o captcha/phone verification or whatever).
We might find out later this year if some site starts blocking @icloud.com but keeps allowing @private.icloud.com.
It does, and I know a friend of a friend who uses them for Walmart accounts to bot pokemon drops. Those and office aliases.