Nestjs instantiate class of services in useFactory

Question

I was checking one of the nestjs module's source code (nestjs-firebase-admin) and I saw something weird. In this line:

private static createProviders(app: admin.app.App): Provider<any>[] {
  return PROVIDERS.map<Provider>((ProviderService) => ({
    provide: ProviderService,
    // here, instantiate each service class
    useFactory: () => new ProviderService(app),
  }));
}

Why do they instantiate each service class? This should be handled by nest core. As I know we just instantiate plain js classes when wrapping up another native plugin. But these are nestjs services. So we should not instantiate them manually. Any idea?

Note: PROVIDERS defined as (all of them are services):

const PROVIDERS = [
  FirebaseAuthenticationService,
  FirebaseMessagingService,
  FirebaseRemoteConfigService,
  FirebaseDatabaseService,
  FirebaseFirestoreService,
  FirebaseStorageService,
];