I'm not sure what you mean by this; WebKit Tracking Prevention doesn't break third-party login. Third-party login works by handing a token back to the first party, who then stores it directly and validates it on the backend. After the initial login with the third party, the third party isn't involved anywhere that the browser can see. And the initial login happens in a browser window that's navigated directly to the third party, making them the first party for the login form (and therefore able to access any saved login data from previous logins).

In fact, this WebKit Tracking Prevention Policy explicitly states that third-party login is implied consent for the third party to identify the user as having the same identity in these multiple places.

Sign In With Apple has the same level of access as any other OAuth-based federated login scheme. The popular login services from Facebook and Google work fine in Safari. As others have mentioned, the way they are built does not require cross site tracking.