@User the docs for handling the callback on the backend are wrong. Apologies for that. What you should actually do is:

ThirdPartyEmailPasswordNode.init({
  signUpFeature: {
    handlePostSignUp: async (user, context) => {
      let {id, email} = user;
      if (context.loginType === "emailpassword") {
        let formFields = context.formFields;
        // TODO...
      } else {
        let thirdPartyAuthCodeResponse = context.thirdPartyAuthCodeResponse;
        // thirdPartyAuthCodeResponse is the response from the third party login provider that contains the access / refresh tokens sent by them..
      }
    }
  }
});