Android Studio - How to fix back button still working after using flag_activity_clear_task | flag_activity_new_task?

I am trying to add a logout/signout function to my app. I am following this video on youtube to do that. Logout/signout function is working just fine but after log out in the login screen I can go back to the app. I am still not trying using the onBackPressed() method. Maybe I can work to prevent the back action, but I am just curious why my app not working.

Here's the sign out fragment code

public class SignOutFragment extends Fragment {

    private static final String TAG = "SignOutFragment";

    //firebase
    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;

    private ProgressBar mProgressBar;
    private TextView tvSignout, tvSigningOut;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_signout, container, false);
        tvSignout = (TextView) view.findViewById(R.id.tvConfirmSignout);
        mProgressBar = (ProgressBar) view.findViewById(R.id.progressBar);
        tvSigningOut = (TextView) view.findViewById(R.id.tvSigningOut);
        Button btnConfirmSignout = (Button) view.findViewById(R.id.btnConfirmSignout);

        mProgressBar.setVisibility(View.GONE);
        tvSigningOut.setVisibility(View.GONE);

        setupFirebaseAuth();

        btnConfirmSignout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "onClick: attempting to sign out.");
                mProgressBar.setVisibility(View.VISIBLE);
                tvSigningOut.setVisibility(View.VISIBLE);

                mAuth.signOut();
            }
        });

        return view;
    }

     /*
    ------------------------------------ Firebase ---------------------------------------------
     */

    private void setupFirebaseAuth(){
        Log.d(TAG, "setupFirebaseAuth: setting up firebase auth.");

        mAuth = FirebaseAuth.getInstance();
        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();

                if (user != null) {
                    // User is signed in
                    Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());
                } else {
                    // User is signed out
                    Log.d(TAG, "onAuthStateChanged:signed_out");

                    Log.d(TAG, "onAuthStateChanged: navigating back to login screen.");
                    //Intent intent = new Intent(getActivity(), LoginActivity.class);
                    Intent intent = new Intent(getContext(),LoginActivity.class);
                    intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK| Intent.FLAG_ACTIVITY_NEW_TASK);
                    startActivity(intent);
                    getActivity().finish();
                }
                // ...
            }
        };
    }

    @Override
    public void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthListener);
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mAuthListener != null) {
            mAuth.removeAuthStateListener(mAuthListener);
        }
    }
}