They could revoke your keys (in theory). I don't think they would though

You misunderstood; I'm saying Apple couldn't coerce a website into offering Sign in with Apple as an option, like they can with an app on the App Store.

To me, this is a feature: I don’t want to have a million passwords because some random developer doesn’t like my preferred authentication method. As a user, I prefer to pick a trusted authentication provider and use it everywhere because, in the end, it’s a lot simpler for me: I don’t need to remember half a million username/email/password combinations, I just need to be able to authenticate myself to Apple.

Developers, web developers especially, often seem to forget that the browser is a _user_ agent. Instead, they force their aesthetic and functional preferences on me, when I have chosen fonts, colors, interactions and systems that I prefer to be able to use wherever possible.

And they are saying that apple can do that. By threatening to revoke payment keys.

What kind of payment keys would they revoke for a website that does not offer ‘Pay with Apple Pay’?