Suitable way for Validating JPA Entities in Spring Boot
The approach is ok, although it depends on the context. If the application receives AppUserDto in the controller layer, doing the validation in the service layer might be too late. Moreover, the service layer will be dependent on the DTO.
If AppUserDto is received in the controller layer (e.g. from a REST API) a good approach is validating the request in the controller layer, mapping the DTO to the entity, and passing it down to the service layer:
@RestController
public class UserController {
    //...

    @PutMapping(value = &quot;/user&quot;)
    public void updateUser(@RequestBody @Valid AppUserDto appUserDto) {
        AppUser appUser = toEntity(appUserDto);
        appUserService.update(appUser);
    }

    //...
}

After the validation with @Valid, if there are no errors, toEntity will map the DTO to the entity. The mapping can be done manually or using ModelMapper. More info on this approach here.
In general, DTO validation should be done as soon as possible to prevent internal components to work with invalid objects (as mentioned in the article you linked). Additionally, mapping the DTO to the entity keeps the service layer independent of the particular request so that it can be better reused and tested.