Batch operation isn't called
When you run a batch the URL automatically changes to /batch...at that path, Drupal doesn't know to include your module_definition.inc file. It'll only process the menu hook when the path is 'some-pretty-url', so module_definition.inc will only be included then.

If you move your batch definition into the module file it'll start working, and the file you've told the batch system to find the processing function in will be included as you'd expect, because now Drupal knows it needs to be included.