Expose Custom DB Tables to Drupal Views Error
The tutorial you linked to implements hook_schema() which means Drupal will want to take care of the table implementation for you.

I don't know this for definite but I think Views requires that the table be under Drupal's control when you're exposing custom data.

I don't know of a way to tell the installer to ignore the schema installation, so the simplest thing to do would probably be to rename the custom table temporarily, install the module (allowing Drupal to create the table), then import the data from the temp table into the newly created one.

If you then uninstall the module (not just disable), your custom table and the data will be lost so it's worth keeping a backup if you think that's likely to happen.