One other thing that seems powerful is that users that use Sign In With Apple have some guarantee of quality; with Apple using FaceId to authenticate, there's some amount of guarantee that you're not a bot.
I think this is something that people are missing when they suggest services will just block the Apple relay address.
Of course they wont. They still want the business and as you've pointed out, these accounts will be in a different customer engagement category. They are almost certainly real people and they a lot of value to marketers, even is you don't have all of their other personal details.
