JUnit CRUD: Why assuming that you use findById instead of existsById?

Question

I took the following code from https://codefiction.net/unit-testing-crud-endpoints-of-a-spring-boot-java-web-service-api

@RunWith(MockitoJUnitRunner.class)
public class DeleteUserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private DeleteUserService deleteUserService;

    @Test
    public void whenGivenId_shouldDeleteUser_ifFound(){
        User user = new User();
        user.setName("Test Name");
        user.setId(1L);

        when(userRepository.findById(user.getId())).thenReturn(Optional.of(user));

        deleteUserService.deleteUser(user.getId());
        verify(userRepository).deleteById(user.getId());
    }
}

In deleteUserService.deleteUser to test if a user exists, we could have also used existsById instead of findById. In this case, the unit test would fail.

I am new to unit testing, but this does not seem right. But also I cannot find a good solution: I mean, when I write the test I should mock all the possible methods that could test the existence? It seems too heavy.