How to integrate the drush help --filter option with your custom module?
I found the answer searching in the code of the drush project:

If the module have less than 4 commands you can't use the drush --filter=MODULE_NAME because your commands are classified as other:

See the function drush_commands_categorize(), and at line 1125 you can see this comment:

  // Post-process the categories that have no title.
  // Any that have fewer than 4 commands go into a section called "other".


I create 3 more commands and now the drush --filter=onlyone command works.