Exactly. Or if you're serious about your website 99 ain't that bad.
Sign in with Apple is a service aimed at consumers, not developers. It isn't great for developers because it's made to decrease tracking, it offers things like convenient redirect emails that forward to your real address. It makes things like marketing harder, so having to pay for it on top isn't very encouraging. Users that want to sign up and are going to be active in your community aren't going to sign up solely because you offer Apple signin, or not sign up because you don't, so the impact of not offering this is effectively zero.
The main grab they have for increasing adoption of this feature is by
forcing apps that use any kind of third-party login (whether that be social login via Facebook/Google/GitHub/etc, or apps handling their authentication using services like Duo) to also support this feature.
I can't imagine even sites that can pay the $99/yr would want to use this too much, since it decreases their control. For your users it's great, though, and it'd be nice to see it in XF, I just don't see many people using it.
Though I wonder if only the paid developer accounts can use this, and not the free developer account as well? Apple offers a free developer account so you can use Xcode, create signing certificates and deploy your app to devices you control, etc. At a quick skim I couldn't see them mentioning the requirement to use this.