I checked your example and found that you're trying to define Validator as a nested component inside the "SimpleMDE" component.
While it's not possible to use dxValidator inside a custom third-party component as is, you can place it in a stand-alone component as demonstrated at Validate a Custom Value
However, I checked this approach and found that it doesn't work properly and created a separate report for further research:
It's impossible to define Validator as a stand-alone component in React
As a workaround, you can create dxValidator using our built-in API: 3rd-Party Frameworks Integration API.
I've attached the modified example to demonstrate this idea.