There's the private relay emails that are used via Sign in with Apple, and then there are the Hide My Email addresses, which are just random emails tied to your account. The private relay ones it can detect because they use an "@ privaterelay dot appleid dot com" domain, but the Hide My Email addresses just use "@ icloud dot com."
So Revolut or whatever random site would have no way of knowing the difference between "MyActualEmail @ icloud dot com" or "RandomHideMyAddress @ icloud dot com."
The change Apple was going to make would have allowed those sites to easily differentiate between "MyActualEmail @ icloud dot com" and "RandomHideMyAddress @ hide my dot icloud dot com."